Journal of Building Engineering 58 (2022) 104979
Available online 30 July 2022
2352-7102/© 2022 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Achieving superhydrophobic surfaces with tunable roughness on building materials via nanosecond laser texturing of silane/
siloxane coatings
Luis A.M. Carrascosa
a,c, Rafael Zarzuela
a,c,*, Marta Botana-Galvín
b,c, Francisco J. Botana
b,c, María J. Mosquera
a,c,**aTEP-243 Nanomaterials Group, Department of Physical-Chemistry, Faculty of Sciences, Universidad de C´adiz, 11510, Puerto Real, Spain
bDepartment of Materials Science, Metallurgical Engineering and Inorganic Chemistry, Engineering School, Universidad de C´adiz, 11519, Puerto Real, C´adiz, Spain
cIMEYMAT: Institute of Research on Electron Microscopy and Materials, Universidad de C´adiz, 11510, Puerto Real, C´adiz, Spain
A R T I C L E I N F O Keywords:
Organically modified silica Superhydrophobicity Laser
Top-down Building material Roughness
A B S T R A C T
In this work, we employ a versatile laser-based top-down approach that allows to create super- hydrophobic and hydrophobic surfaces, with controlled roughness and wetting properties, on marble and potentially other building materials. The process involves two stages: (1) application of an organically modified silica coating to reduce surface energy. (2) Controlled texturing of the coating by ablation using a nanosecond-pulse laser. In general terms, at higher laser fluence (energy per unity of area), the contact angles increased from 110◦of the non-textured surface to values around 155◦, following a nearly linear correlation with the measured roughness values.
Starting from fluence values of 154 J cm−2, the surfaces displayed water repellence (hysteresis
≤10◦) and the micrographs showed the formation of sub-micrometric structures on top of the micro-roughness by melting and re-deposition of the coating material, suggesting the formation of a Cassie-Baxter wetting regime. Ablation at lower fluences created a random micro-roughness, leading to static contact angles of 135–145◦but no water repellence, which is indicative of a Wenzel wetting regime. At the highest fluence values tested, the increasing trends respect roughness and hydrophobic/water repellent properties are inverted due to the damages suffered by the coating. In terms of durability, the coating demonstrated a good adhesion to the stone surface, maintaining its superhydrophobic properties after repeated cycles of an “adhesive tape test”. The sand falling test showed that water repellence is relatively sensitive to abrasion, although the hydrophobic character of the coating is maintained.
1. Introduction
Prolonging the service life of building materials under aggressive environments and working conditions has become a priority for the construction market, as repair and regular maintenance actions are associated to significant costs and environmental impact.
Among the usual decay agents, water is often considered the most aggressive due to its role in a wide variety of degradation
* Corresponding author. TEP-243 Nanomaterials Group, Department of Physical-Chemistry, Faculty of Sciences, Universidad de C´adiz, 11510, Puerto Real, Spain.
** Corresponding author. TEP-243 Nanomaterials Group, Department of Physical-Chemistry, Faculty of Sciences, Universidad de C´adiz, 11510, Puerto Real, Spain.
E-mail addresses: [email protected] (R. Zarzuela), [email protected] (M.J. Mosquera).
Contents lists available at ScienceDirect
Journal of Building Engineering
journal homepage: www.elsevier.com/locate/jobe
https://doi.org/10.1016/j.jobe.2022.104979
Received 7 March 2022; Received in revised form 28 April 2022; Accepted 14 July 2022
processes [1], either by direct action (e.g. freeze-thaw cycles, lixiviation) or as a vehicle for other agents (e.g. soluble salts, bio- deteriogens, acid rain). Calcium-rich building materials, such as limestone, marble or concrete are especially susceptible to acid rain, especially in urban and industrial environments, whose effects include solubilization, sulfate efflorescence or contribution to the formation of black crusts.
For this reason, there has been a growing interest for the development of hydrophobic protective treatments, which decrease the water ingress in the material. More recently, superhydrophobic treatments have been extensively researched for their water repellent and self-cleaning properties. Superhydrophobic surfaces are characterized by water static contact angle (SCA) values above 150◦and hysteresis between advancing and receding contact angle values (CAH) values lower than 10◦ [2], which is a measurement of the repellence phenomenon. In order to obtain superhydrophobic surfaces, it is often necessary to simultaneously decrease the surface free energy and to produce a regular micro/nano-roughness [2,3]. Under these conditions, the surface reaches a Cassie-Baxter wetting state [3], characterized by the entrapment of air pockets between its roughness features and a decrease in the surface-water contact area.
The Cassie-Baxter model predicts that the static contact angle increases proportionally to the contribution of the air-solid interface and the lower interaction forces allow water to easily roll over the surface. In contrast, hydrophobic surfaces with an irregular or flat roughness profile are defined by a Wenzel wetting regime [4], where water impregnates the roughness features and is pinned to the surface. The static contact angle of a hydrophobic surface defined by a Wenzel regime increases with its roughness, proportionally to the developed/projected area ratio.
Several approaches have been reported in literature to produce the required surface roughness for a Cassie-Baxter state, which can be divided in two categories [5]: (1) bottom-up methods, where the roughness is created from smaller units (monomers, oligomers, nanoparticles …) by polymerization, aggregation or self-assembly processes. (2) Top-down methods, where the roughness is created by the controlled removal of material from the bulk. Examples of top-down methods include lithography, plasma etching and laser ablation [6].
In the specific case of building materials, most efforts have been focused on the use bottom-up methods [7,8], mainly through the use of functionalized nanoparticles and/or their combination with polymer or sol-gel (e.g. organically modified silica gel, fluo- roalkylsilane) coatings, due to their simplicity and suitability for on site applications. Top-down strategies, however, have barely been researched for this application despite their potential advantages, such as: (1) the possibility to control of the surface morphology, (2) lower susceptibility of the process to external factors (e.g. temperature, moisture) or (3) the reduced use of potentially contaminant or hazardous reagents (e.g. organic solvents, micronized silica, fluorinated resin precursors). Although these strategies have been usually considered too complex for their practical use on building materials, the development of portable technologies and equipment designed for treatment of large areas, may place them as a viable alternative to traditional bottom-up methods.
Among the top-down methods, laser texturing has gained attention during the last years for a wide variety of applications [9,10].
This strategy has been successfully applied for controlling the roughness of substrates such as metals, semiconductors or polymers, with applications in nano/microfluidics, optofluidics, lab-on-chip technology, fluidic microreactors, biochemical sensors, biomedi- cine, and thermal management. Reports on the use of laser for controlled texturing of stone and other building materials, however, are scarce. The use of portable laser systems has been reported for the removal of stains and biological patinas on building materials [11, 12]. L´opez et al. [13] studied the use of femtosecond laser to create nano/micro-structured roughness on different ornamental stones and elucidated the correlations between laser parameters, roughness and wetting properties. The authors found that texturing could increase or decrease hydrophobicity depending on the stone properties (surface defects, composition, homogeneity), which determine whether the surface behaves in a Cassie-Baxter or Wenzel state respectively. In order to consistently achieve superhydrophobic sur- faces on building materials, however, it is desirable to condition the surface so that it has a low surface energy and homogeneous composition.
In this work, we studied a two-stage methodology to consistently create superhydrophobic coatings with self-cleaning performance on building materials. The first stage consists on the application of a ormosil-based primer coating via a sol-gel method, which is intended to decrease the surface energy and create a homogeneous starting surface. The ormosil coating was prepared according to a previously reported surfactant assisted method [14] that promotes a proper interaction with carbonate building materials and gen- erates a mesoporous xerogel that has a relatively low impact over the water vapor permeability of the substrates, with reported re- ductions below 30% (tested by ASTM E96 96 M) on different materials.
The second stage consists on the ablation of the ormosil coating using a nanosecond laser, in order to modify the surface topography and the wetting properties. Specifically, the effect of varying laser fluence (applied energy per area) was studied to verify whether the methodology allows to control the surface total roughness and topography and their effect on the formation of Wenzel or Cassie-Baxter wetting regimes. The methodology is demonstrated on marble surfaces, though the versatility of the sol-gel technique chosen to apply the primer coating suggests that this process would be potentially applicable to different building materials.
2. Experimental
2.1. Synthesis and application of the hydrophobic primer coating
The hydrophobic primer coating was prepared by a sol-gel route according to a procedure developed in a previous work [14].
Specifically, an oligomeric ethoxysilane (TES40 WN, from Wacker Chemie AG), and polydimethylsiloxane (PDMS from ABCR) were mixed with n-octylamine and water (89.67 TES40, 10.00 PDMS, 0.08 n-oct. and 0.25 H2O %v/v) using an ultrasound probe (Bandelin®
Ultrasonic HD3200) at 1 W/ml during 10 min.
The prepared sol was sprayed on marble slabs, obtained from Macael Quarry (Spain), by using an air-gun (pressure 3 bar) (10 × 10
×1 cm3). The excess product was removed by blowing air (pressure 2 bar). Afterwards, the samples were stored for one month at room
conditions (20 ◦C, 40% RH) to allow polymerization and dying of the gel.
2.2. Texture modification by laser ablation
Laser ablation of the surfaces (in 2.5 × 2.5 cm2 areas) was carried out using a Nd:YAG solid state tripled-frequency laser at 355 nm wavelength and 11 ns pulse duration, model TruMark6350 from Trumpf. At 33 kHz, the maximum nominal power is 5 W, resulting in a pulse energy of 0.15 mJ and pulse power of 13.6 kW. Considering that the beam diameter at the focal length is approximately 10 μm, the maximum fluence of the laser was calculated as 193 J cm−2. All the experiments were carried out at constant pulse repetition rate of 20 kHz, and scanning speed of 50 mm s−1. Different experiments were performed by varying the laser power between 60 and 100%, for a fluence interval between 116 and 193 J cm−2. Correlation between pulse power and fluence is presented in supplementary material (Table S1).
2.3. Evaluation and surface characterization techniques
The wetting properties of the marble surfaces were characterized by measuring the SCA and CAH values of 5 μl water droplets by using a commercial video-based, software-controlled contact-angle analyzer (model OCA 15plus, from Dataphysics Instruments).
Details of the experiment are reported in a previous paper [15].
The self-cleaning performance of the superhydrophobic marble surfaces was evaluated qualitatively by observing their ability to remove stains under a low pressure water stream [16]. Roof tile powder was selected as a model agent to simulate dust build-up on the surface.
The modifications on the surface topography caused by the laser treatment were evaluated by means of two techniques. Surface roughness was evaluated by using confocal optical profilometry. A 3D optical profilometer, model Z-300 from Zeta Instruments was employed. Sa (arithmetic mean surface roughness) were calculated from 2.0 × 2.5 cm2 areas for each treatment. Morphology of the surfaces under study was visualized by Scanning Electron Microscopy (SEM), using a Quanta200 equipment from FEI, working in secondary electrons mode at an acceleration voltage of 30 kV. Prior to the measurement the samples were gold-sputtered with a 5 nm layer.
2.4. Evaluation of durability
Durability of the superhydrophobic (textured) coating was evaluated in terms of its mechanical stability by two different tests, namely: an adhesive tape test and a sand fall test.
The adhesive tape test is a commonly used methodology to evaluate the attachment of coatings to a substrate and their mechanical integrity, and it has been used by several authors as a durability test for superhydrophobic surfaces [17]. The procedure followed was based on a modification of the surface cohesiveness tests used in previous works [15] and involves successive attachment/peeling cycles with a pressure-sensitive adhesive tape. Specifically, a tape with a peeling strength of 8 N/cm (Scotch® Filament Tape Universal, from 3 M) was used in the tests. 1.5 × 3.0 cm2 adhesive tape strips were pressed on the surfaces and swiftly peeled (approx. 7–10 mm/s) at 90◦ angle. The process was repeated up to 70 cycles and durability was assessed by monitoring the contact angles and hysteresis.
The sand fall test simulates the behavior of the coating under aggressive and/or prolonged wearing conditions, which the materials
Fig. 1. (a) Evolution of SCA and CAH values with laser fluence value. Photographs of (b) hydrophobic (Fluence 0 J cm−2) and (c) superhydrophobic (Fluence 174 J cm−2) coated marble slabs immersed in water, showing the entrapment of air pockets. (d) Hydrophobic (Fluence 135 J cm−2) coated marble slabs immersed in water, showing an intermediate state. (e) Behaviour of the water droplets deposited on the hydrophobic (Fluence 0 J cm−2) and (f) superhydrophobic (Fluence 174 J cm−2) surfaces.
may find in some applications (e.g. floors, outdoors on windy locations). For this test, the methodologies found in literature for superhydrophobic surfaces were adapted [17]. Specifically, the test was performed using sieved (quartz) sand with a 0.1–1 mm diameter. The sand was dropped at a 456 g/min rate from a 20 cm height over a ~3 cm2 area. Durability was assessed by monitoring the contact angles and hysteresis at different cumulative sand weights.
3. Results and discussion
3.1. Evaluation of the performance and surface characterization
In order to test the relationship between the wetting behaviour of the textured surfaces and the applied laser fluence, SCA and CAH values on the treated marble slabs were evaluated (see Fig. 1a).
The non-textured surface displayed a hydrophobic character (as shown by the droplet photographs in Fig. 1e) with SCA value of 112◦, due to the low surface free energy of the hydrophobic primer coating [14]. The effect of the roughness induced by the laser treatment can be clearly seen by a significant increase in the SCA values up to values in the 140–155◦range and the nearly spherical shape of the deposited droplets (Fig. 1f). Furthermore, a positive linear trend (coefficient of determination R2 >0.9) was observed between the SCA and the laser fluence values, and angles characteristic of superhydrophobic surfaces (>150◦) were obtained at 150 and 174 J cm−2. This trend, however, was inverted at highest fluence value (193 J cm−2), where a decrease of the SCA was observed.
A similar trend is observed for the repellence of the surfaces, characterized by the CAH values. Non-texturized surfaces showed a high CAH value (c.a. 30◦), with no repellent properties, and a linear decrease of the CAH with laser fluence was observed. In accordance with the static angles, the most effective treatments were obtained for laser fluences of 154 and 174 J cm−2, which dis- played CAH below 10◦that, along with the higher (>150◦) SCA values, confirmed the superhydrophobic character of these surfaces [2]. This behaviour suggests the existence of a Wenzel state in non-texturized samples whereas laser treated surfaces transition to a Cassie-Baxter regime as fluence increases. A further increase of laser fluence, however, caused the loss of the repellent properties, manifested by the CAH change from 4◦(174 J cm−2) to 12◦(193 J cm−2), which is characteristic of the phenomenon known as petal rose effect [18], where water partially penetrates into the grooves of a highly hydrophobic surface, and water droplets get pinned to the surface. The apparition of this effect suggests chemical changes and/or surface alterations produced by the high irradiated energy, which can perturb the Cassie-Baxter state.
The contact angles obtained with this laser ablation strategy are comparable previous reports by the authors [19,20] where superhydrophobic coatings with SCA ~150–155◦and hysteresis ~5–7◦were created on sandstone by incorporating 50–100 nm SiO2
nanoparticles to an ormosil. Other authors have obtained superhydrophobic coatings on marble [8] with SCA in the 155–160◦range by incorporation of nanoparticles of different composition (SiO2, TiO2, Al2O3) and sizes in siloxane-based sols and composites with organic polymers (e.g. PMMA, PS), showing that smaller nanoparticles allow obtaining superhydrophobic surfaces with a hysteresis below 5◦at concentrations below 2%. Surfaces possessing remarkably high SCA (~170◦) and sliding angle <5◦have also been ach- ieved on marble by application of a hybrid TEOS/fluoroalkylsilane coatings [21]. The use of long-chain fluorinated compounds commonly yields surfaces with higher contact angle due to their lower surface energy respect hydrocarbon chains, even when the roughness features are not as defined. Liu et al. [22] report coatings for building materials with SCA ~160◦and extremely low sliding angles (<1◦) by combining fluorosilicon resin with hydrophobic HDMS-functionalized diatomaceous earth, which possess an inherent hierarchical roughness.
Qualitative evidences of the transformation from Wenzel to Cassie-Baxter wetting states due to the laser treatment are observed visually by the interaction of air with the surface after immersion in water (Fig. 1b–d), as the Cassie-Baxter is characterized by the entrapment of air between the roughness protrusions [3,10]. In general, the observations are in agreement with the hysteresis values, which act as an indicator of the wetting state for chemically homogeneous hydrophobic surfaces. Specifically, the (superhydrophobic) surfaces textured at 174 J cm−2 (Fig. 1c) and 154 J cm−2, which presented a hysteresis below 10◦, manifested a “mirror-like” aspect due to the entrapment of an air layer in-between the water and solid. In contrast, the surfaces textured at 116 J cm−2 or non-textured (Fig. 1b), whose hysteresis was above 15◦, showed no visible air layers or bubbles, in line with the total impregnation of the roughness features described in the Wenzel model [4]. The surface textured at 135 J cm−2 (Fig. 1d) displayed the entrapment of air bubbles, but to a lesser extent compared to the superhydrophobic surfaces, suggesting that it behaves as a transition state between Wenzel and Cassie-Baxter. Similar observations can be made for the surface textured at 193 J cm−2 (Fig. S1), though its behavior is closer to a Cassie-Baxter regime.
A quantitative estimation of the amount of trapped air can be obtained from the contact angle values and roughness values using the Cassie-Baxter equations:
cos θCB= − 1 + f + rfcos θo
Where θCB is the static contact angle of the Cassie-Baxter surface, θo is Young’s contact angle, f is the fraction of the projected area wetted by the liquid and rf is the roughness ratio of the wetted area. The θo value was approximated as the contact angle of the non- textured surface, considering the SEM images (Fig. 4a) showed an almost flat aspect. The values of rf were estimated using Wenzel equation, assuming r = rf for the calculations on the superhydrophobic surfaces.
cos θW=r cos θo
The amount of air trapped in the protrusions, expressed as %Area is calculated as (1-f)⋅100. The results obtained are as presented in Table 1.
As it can be observed, the quantified %Aair values are in agreement with the qualitative observations (Fig. 1 and Fig. S1), in the sense that the water-air interface represents ~80% of the contact area for the superhydrophobic surfaces. This value is markedly lower for the surface textured at 135 J cm−2, which behaves as an intermediate state. It should be noted that the Cassie-Baxter equation is not fully applicable to this surface, so the estimation may be inaccurate.
The self-cleaning performance of the superhydrophobic surfaces was evaluated for samples texturized with a fluence of 174 J cm−2, which showed the highest SCA and repellence. Specifically, roof tile powder was deposited on the untreated and treated marble and cleaned with a low-pressure water stream (A video recording of the experiment is available in Supporting Information, Video S1). For the untreated sample (Fig. 2 a,c), the water is able to wet the surface and spread the powder, leaving a visible trail. In the case of the superhydrophobic sample (Fig. 2 d,e), however, the low interaction causes the water to easily roll off the surface while carrying the powder, thus resulting in an effective removal of the stain.
Supplementary data related to this article can be found at https://doi.org/10.1016/j.jobe.2022.104979.
As previously commented, the differences in the wetting properties of the studied surfaces result from a combination of their surface energy—constant for all cases—and the influence of their topography (i.e. roughness and morphology) [2,19]. Results of the textural analysis are presented in Fig. 3 and Fig. 4, showing surface roughness maps and SEM images, respectively, of the surfaces texturized at different laser fluence values.
The influence of the laser treatment on the primer coating become texture is shown by the profilometry maps and the mean surface roughness (Sa) values (Fig. 3), which showed an evident increase of the roughness respect to the non-texturized surfaces. The non- texturized surface presented a relatively smooth profile (Sa = 6.5 μm), practically without any defining topographical features, which is consistent with the typical morphology of the primer coating applied observed in other works [19]. At the lower laser fluence (115.81 J cm−2) values, the average roughness barely changed (Sa =6.6 μm), but the surface presented a profile defined by plateaus and shallow valleys, indicating that the laser caused a mild ablation of the material. As laser fluence increases, the height differences became more pronounced (higher Sa) and the surfaces presented an aspect characterized by the presence of more tightly packed protrusions and valleys, evidencing that a higher energy was necessary to promote effective changes to the material. A closer analysis of the Sa values showed a linear trend (coefficient of determination R2 >0.95) correlating the fluence (in the 115–173 J cm−2 range) with the Sa values, in line with the similar trend observed for the contact angle (SCA, CAH) values (Fig. 1a). Similarly, the surface textured at a higher fluence (193 J cm−2) was defined by similar profile but with a lower Sa value, suggesting that the laser caused damages in the surface features. In general, these results demonstrate that it is possible to control the average roughness by modifying the laser parameters.
This almost linear dependence of SCA value with roughness is in good agreement with the Wenzel model, which stablishes that, for an inherently hydrophobic material (i.e. SCA >90◦on an ideal flat surface), the contact angle will increase proportionally to a factor defined by the surface roughness [4]. Although roughness values can be related with SCA, the Sa parameter and the Wenzel model cannot explain the superhydrophobic performance shown by the surfaces texturized at higher fluence values. An explanation for this of behaviour needs to account for the surface morphology at the sub-micrometric level and hierarchical structures [23] (see Fig. 1c).
Thus, the sub-micrometric morphological features of the surfaces under study were characterized by SEM. Fig. 4 shows the mi- crographs of the texturized and non-texturized surfaces at different fluence values.
For the non-texturized coated marble (Fig. 4a), a practically smooth surface was observed, along with sparse defects in the coating, which may explain why Ra values are similar to the surface textured at lower fluence (see Fig. 3a). This smooth surface would explain the relatively low SCA value (112◦), since as reported in the literature [2,24], the maximum SCA value achievable for an ideal flat surface would be 120◦[25]. Regarding the texturized surfaces, those irradiated with fluence values of 116 and 135 J cm−2 (Fig. 4b and c, respectively) showed a heterogeneous rough surface, where the laser ablation caused delamination-like removal of the material. An increase of surface roughness with the increase of laser fluence value, as well as the appearance of some (poorly defined) sub-micrometric bumps can be observed, which is in agreement with the data obtained from profilometry measurements. This situ- ation fits the definition of a Wenzel surface, characterized by the presence of roughness features separated by large pitches in random arrangements.
The surfaces textured at higher fluence, on the other hand, showed a hierarchical structure, characterized by the presence of submicron-scale (<0.5 μm) ellipsoid particles of regular size covering the larger protrusions. These micrometric particles originated from melting of the coating material and its subsequent re-deposition on the surface after cooling down, similarly to the observations made by other authors [26]. As explained in the previous sections, this hierarchical roughness promotes the creation of a Cassie-Baxter state, giving rise to superhydrophobic properties. In the case of the surface treated with the highest fluence (193 J cm−2, Fig. 4f), the Table 1
Influence of laser fluence variations of the average roughness (Ra), roughness ratio of the wetted area (r) and the amount of air trapped in the roughness valleys (%Aair) according to Cassie-Baxter equation.
Fluence (J⋅cm−2) Ra (μm) r %Aair
0 6.5 1.0 0
116 6.6 1.7 an/a
135 15.5 1.8 a59
154 18.5 2.1 78
174 25.3 2.1 82
193 20.8 2.0 74
aEstimations may be inaccurate, as the Cassie-Baxter regime does not fully apply to these surfaces.
excessive energy damaged the previously mentioned structures, as manifested by decreasing number of particles, irregular size and the loss of their ellipsoid shape.
The explanation of the influence of topography on the formation of a Cassie-Baxter or Wetting state is closely related to the air entrapment capacity (see Table 1), which is ultimately defined by the activation energy of the wetting transition (i.e. Cassie-Baxter to Wenzel) [23]. The Cassie-Baxter state is less energetically favorable than the Wenzel state, but becomes (meta)stable when the activation barrier of the transition between wetting states (which involves water displacing the air between the roughness protrusions) overcomes the pressure exerted by the water droplet. This energy barrier becomes higher when the surface energy is decreased, as this makes the solid-water interface less stable. However, the contribution of surface energy alone is insufficient to stabilize the Cassie-Baxter state, which is more dependent on surface geometry. Of relevance to the observations from this work, there are two known aspects that increase this activation energy, namely: (1) the presence of taller protrusions separated by relatively narrow (nano and sub-micrometric) pitches [20,23]. (2) For a hydrophobic material, hierarchical roughness combining micrometric details with smaller protrusions to their sides increases this energy barrier due to the larger area of the hydrophobic surface that should be wetted for the transition to occur.
The non-textured coating and the coating textured at lower fluence presented no defined features at the sub-micrometric scale, and the micrometric roughness is characterized by wide valleys, both of them conditions which favor wetting of the protrusions. The surfaces textured at 154 J cm−2 and 174 J cm−2 fulfill the two conditions that increase the energy barrier between wetting states (i.e.
Fig. 2. Cleaning test with low pressure water on the marble surfaces soiled with dust. (a–c) Untreated specimen. (d–f) Treated specimen texturized at 174 J cm−2 fluence. A full video of the test is available in supplementary material.
Fig. 3. Topographical maps obtained by confocal optical profilometry of the coated marble surfaces texturized at different laser fluences: a) Non-texturized, b) 116.
J⋅cm−2, c) 135 J cm−2, d) 154 J cm−2, e) 174. J⋅cm−2, 193 J cm−2. The measured surface was in all cases 2.0 × 2.5 cm.
hierarchical roughness and sub-micrometric features separated by narrow pitches), in combination with their higher micrometric roughness (observed by profilometry). The surface textured at 135 J cm−2 does not display regular sub-micrometric features save for some barely noticeable bumps (observable by SEM), but the profilometry indicates that the large protrusions are taller and separated by smaller pitches than the surface textured at lower fluence. This may explain why the surface seems to behave as a transition state.
For the surface textured at 193 J cm−2, the SEM images show how the sub-micrometric features become less defined.
With regards to the relationship between laser parameters and the effects on the coating morphology, scientific literature states that the effect of laser ablation over a surface depends on three main factors: (1) The number of pulses per area [26] (related to pulse frequency, spot laser diameter and scanning speed). (2) The ablation threshold [27], which depends on surface properties such as composition or roughness. (3) The fluence value of the laser radiation. Under the experimental conditions used for this work, the first two variables are assumed as constant. Regarding to the first factor, the pulse repetition ratio (20 kHz), spot laser diameter (10 μm) and scanning speed (50 mm s−1) were maintained constant for all the texturized surfaces. Under this conditions, 4 consecutive laser pulses would overlap on any given spot. Thus, a cumulative effect due to the overlapping of laser pulses is expected [10,26]. On the other hand, the homogeneous composition and texture of the primer coating (Figs. 3a and 4a), ensure that the ablation threshold remains constant between different areas of the specimens.
Similar effects on topography after ablation at different laser fluences have been reported by other authors. According to the literature [28,29], fluence values below the ablation threshold produce deformations and holes on the surface, which in turn lead to a moderate increase in roughness. Similar results were observed in the present work for the lowest fluence values, where an increase in surface roughness was observed in (Fig. 3b and c) and SEM (Fig. 4b and c) images showed deformations of the coating. On the other hand, for fluence values exceeding this threshold, the generated heat causes melting and rapid re-solidification of the surface material, which produce its deposition in the form of nanometric or micrometric particles near the edge of the laser spots [30]. It should be noted that the edges of the laser spots are not clearly defined in the surfaces under study due to the overlapping of pulses, although the particles are observed (Fig. 4d and e). At extreme fluence values, heat dissipation rate is not quick enough and the laser destroys the previously formed surface features, which subsequently re-arrange in random patterns [31], in line with the changes observed in Fig. 4f.
Comparable observations to the results obtained in the present work have been reported for other materials texturized by laser.
Yoon et al. [32] reported the fabrication of superhydrophobic PDMS sheets by laser texturing with a femtosecond laser (150fs) at a constant pulse repetition ratio of 1 kHz. The increase in fluence (from 3.4 to 4.9 J cm−2) promoted the deposition of nanoparticles onto the ablated surface, giving rise to a Cassie-Baxter surface topography with superhydrophobic performance. Farshchian et al. [33]
produced superhydrophobic grid patterns on PDMS sheets by nanosecond (25 ns) laser texturing at a constant pulse repetition rate of 10 Hz and 5 J cm−2 fluence value, also resulting in the deposition of nanoparticles and creation of hierarchical roughness.
It should be noted that, although the observed effects are similar, a direct comparison of the values may prove difficult to establish due to the different laser and material parameters. In the aforementioned works, PDMS sheets were employed as substrate for pro- ducing the superhydrophobic surfaces. Specifically, a 10:1 mixture by weight of PDMS base/curing agent (tetraethoxysilane) was prepared. Although these components are similar to the primer coating used in our work, the lower PDMS/ethoxysilane oligomer employed (c.a. 1:10) produces a highly reticulated silica backbone, unlike the mostly linear structure of PDMS sheets. This difference Fig. 4. SEM images of the coated marble surfaces texturized at different laser fluences: a) Non-texturized surface, b) 116 J cm−2, c) 135 J cm−2, d) 154 J cm−2, e) 174 J cm−2, 193 J cm−2.
in reticulation explain the higher fluence required to surpass the ablation threshold (between 115 and 150 J cm−2) compared to that used in the aforementioned works (around 5 J cm−2).
In general terms, it has been shown that texturization by laser under the appropriate parameters can create a hierarchical profile by simultaneously increasing the micro-roughness while forming smaller structures deposited on the protrusions. By comparison, the superhydrophobic surfaces created by coatings containing nanoparticles [8,19,20] tend to display a high regular roughness on the sub-micron scale—corresponding to the individual particles or their clusters—but usually decrease the micro-roughness after depositing between the larger protrusions of the substrate, which may limit their effect on substrates with a high macro-roughness and surface porosity. The use of particles with inherent hierarchical structures—such as diatomaceous earth—has been reported to improve water repellence [22] when the coating process is optimized to ensure they are exposed on the surface.
The methodology used in this work shows the potential of using laser ablation to achieve a better control of the coating roughness and topography and ultimately improve its superhydrophobic properties by fine optimization of the parameters (i.e. scan speed, scan pattern, pulse frequency, power, wavelength). However, the obtained results suggest some limitations with the equipment used that would make difficult to achieve extremely high contact angles (≥170◦), namely: (1) while average roughness (Ra) is increased with laser fluence, there is a limit where the laser energy starts causing damages to the coating, as the generated heat cannot be dissipated between pulses. (2) As the sub-micrometric structures are created by melting and quick re-deposition of the coating, their size is presumably limited by the laser spot size and pulse frequency. Two potential ways to address these limitations are the use of femtosecond lasers, which have been reported by several authors [6] to allow a fine control of the roughness features on different substrates, or the post-treatment of the textured surface with fluorinated polymers [34], which have a very low surface energy and simultaneously circumvents the organic component decomposition by effect of laser irradiation.
3.2. Evaluation of durability
The adhesion of the superhydrophobic coatings to the substrate is an important requirement for their application to building materials, which are usually exposed to aggressive conditions during their service life. In order to test this factor, the sample texturized with a fluence value of 173 J cm−2, which presented the best superhydrophobic performance, was tested to determine adhesion of the coating and its mechanical integrity through the adhesive tape test, and its resistance to wearing was evaluated through a sand falling assay.
The adhesive tape test is useful to estimate the attachment of the coating to the substrate and its mechanical integrity under decay processes that cause detachment or delamination-like damages (e.g. differential thermal expansion, hydric swelling, ice damage). The evolution of SCA and CAH values with the attach/detach cycles is shown in Fig. 5.
It can be observed that, during the first 20 attach/detach cycles no significant changes in SCA and hysteresis values occurs, con- firming a high adhesion of the primer coating to the marble surface and the sub-micrometric particles generated by laser ablation. After 30 cycles, a slight decrease in SCA can be observed, although the hysteresis values are not significantly affected and the water repellence is maintained even after 70 cycles. This suggest that, any modifications to the surface caused by the tape detachment may have had a minor impact on the sub-micrometric roughness features, and the decrease in SCA is likely associated to minor modifi- cations of the larger protrusions or even contamination from the adhesive tape.
In other terms, the results imply that the Cassie-Baxter state was maintained through the test due to the interaction of the ormosil coating with the marble surface and the integrity of the formed xerogel. This result is in line with previous works of the authors [20, 35], which confirmed the high adhesion of the xerogel prepared according to a similar synthesis route to different stone substrates (sandstone and limestone) due to a combination of chemical interactions and the crack-free structure of the ormosil promoted by the use of a surfactant on its synthesis, which increases mechanical stability of the coating material.
The evolution of the wetting properties during the sand falling test (Fig. 6) offer an estimation of the performance of the super- hydrophobic coating under heavy wearing conditions.
As it can be observed, the abrasion has a clear effect on the wetting properties, manifested in a gradual decrease of the SCA and
Fig. 5. Evolution of SCA and CAH values of the superhydrophobic surface after successive adhesive tape attach/detach cycles.
increase in the hysteresis. After impact from ~1000 g sand, a decrease on the SCA is observed, although the hysteresis remains lower than 10◦and water repellent behavior is maintained. Further abrasion by sand impact leads to a loss of water repellency and the hysteresis increases to 12–18 values, while the SCA decreases almost linearly until reaching ~120◦. Afterwards, the decrease in SCA becomes less steep and reaches the values of the non-textured surface, while the hysteresis reaches a plateau at 17◦. In general terms, it can be affirmed that the coating maintains its hydrophobic character even after intense wearing, although the superhydrophobic properties are less stable.
As previously explained, the Cassie-Baxter state is reliant on the existence of submicrometric roughness features, which are able to entrap air pockets, and is favored by higher protrusions separated by narrow pitches. The impact of sand over the surface generates an intense abrasion which damages the surface and caused changes on its topography. Considering the results, it becomes apparent that the smaller roughness features, which were formed by melting and re-deposition, are more sensitive to physical damages. It is worth noting how, after prolonged testing, the hysteresis is still lower than the value of the non-textured surface, which may be attributed to the creation of a roughness by action of the sand. Nevertheless, this presumed roughness would not be enough to increase the SCA values. As a general conclusion of the sand fall test, improvements of the coating hardness should be considered for applications where a high wearing rate is expected (e.g. floors, outdoor elements on windy environments).
4. Conclusions
The texturization of an ormosil hydrophobic coating, applied on a marble substrate, with a nanosecond laser has been proven as a viable route to modify its roughness and promote the transition from a Wenzel (hydrophobic) to a Cassie-Baxter (superhydrophobic) state. The application of an ormosil as a primer coating ensures that the surface is chemically and morphologically homogeneous and has a low surface energy, making the methodology more consistent and potentially applicable to materials other than marble.
The obtained results showed a direct correlation between the laser fluence and average roughness (as observed by optical pro- filometry), due to the creation of larger protrusions (holes and deformations), which increases the static contact angle in line with the Wenzel wetting model. In addition to this effect, texturing at higher fluence promotes the formation of a secondary sub-micrometric roughness that allows transitioning to a Cassie-Baxter state. Specifically, superhydrophobic surfaces are obtained at fluence values over the ablation threshold (150–170 J cm−2), where the laser is able to cause melting and rapid re-deposition of the coating, generating a sub-micrometric roughness (particles or bumps) over the larger protrusions. This hierarchical structure is able to stabilize the air entrapped in the roughness valleys, preventing the transition from a Cassie-Baxter to a Wenzel state. Although increasing the fluence improves the superhydrophobic properties, a decrease in performance was found at high values, associated to destruction of the sub-micrometric structures due to an inability to dissipate the generated heat.
In terms of durability, the results from the tape test indicate an adequate adhesion of the coating to the marble surface, which is important for preserving the Cassie-Baxter state from degradation processes involving detachment-like damages. Abrasion tests by sand fall showed that the coating reasonably maintains its hydrophobic character (i.e. SCA >90◦), but gradual damages eventually decrease the static contact angle and water repellence of the superhydrophobic surface.
In general, the methodology used in this work shows promising results in terms of versatility for different materials and tenability of the surfaces properties. Further research should focus on studying the influence of different laser parameters (i.e. pulse frequency, scan speed, scan pattern, wavelength, amplitude) over the surface morphology—and its capacity to promote air entrapment and water repellence—or the use of alternative strategies to circumvent the limitations at high fluence or allow a finer control of the sub- micrometric features, such as using femtosecond lasers or post-treatments with fluorinated compounds.
Author statement
Luis A. M. Carrascosa: Writing-Original Draft, Investigation, Formal analysis, Visualization. Rafael Zarzuela: Writing - Original Draft, Formal analysis, Visualization. Marta Botana-Galvín: Investigation, Methodology. Francisco J. Botana: Investigation, Method- ology, Resources. María J. Mosquera: Writing - Review & Editing, Supervision, Funding acquisition.
Fig. 6. Evolution of SCA and CAH values of the superhydrophobic surface during the sand falling test (X axis expressed as cumulative weight). The dotted lines represent the values of the non-textured coating.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This project has received funding from the European Union’s Horizon 2020 - Research and Innovation framework programme under grant agreement No 760858, the Spanish state research agency R&D program 2020 (PID2020-115843RB-I00), and by the department of economic transformation, industry, knowledge, and universities of the Andalusian regional government (FEDER- UCA18-106613) under the European Union’s 2014–2020 ERDF Operational Programme.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.org/10.1016/j.jobe.2022.104979.
References
[1] M.O. Figueiredo, T.P. Silva, J.P. Veiga, Analysis of degradation phenomena in ancient, traditional and improved building materials of historical monuments, Appl. Phys. Mater. Sci. Process 92 (2008) 151–154.
[2] L. Gao, T.J. McCarthy, Wetting 101 degrees, Langmuir 25 (2009) 14105–14115.
[3] A.B.D.D. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc. 40 (1944) 546–551.
[4] R.N. Wenzel, Resistance of solid surfaces to wetting by water, J. Ind. Eng. Chem. 28 (1936) 988–994.
[5] X.-M. Li, D. Reinhoudt, M. Crego-Calama, What do we need for a superhydrophobic surface? A review on the recent progress in the preparation of superhydrophobic surfaces, Chem. Soc. Rev. 36 (2007) 1350–1368.
[6] J. Yong, F. Chen, Q. Yang, Z. Jiang, X. Hou, A review of femtosecond-laser-induced underwater superoleophobic surfaces, Adv. Mater. Interfac. 5 (2018) 1–31.
[7] I. Karapanagiotis, M. Hosseini, Superhydrophobic coatings for the protection of natural stone, in: I. Karapanagiotis, M. Hosseini (Eds.), Adv. Mater. Conserv.
Stone, first ed., Springer International Publishing, Cham, 2018, pp. 1–25.
[8] I. Karapanagiotis, P.N. Manoudis, Superhydrophobic and superamphiphobic materials for the conservation of natural stone : an overview, Construct. Build.
Mater. 320 (2022), 126175.
[9] A.Y. Vorobyev, C. Guo, Direct femtosecond laser surface nano/microstructuring and its applications, Laser Photon. Rev. 7 (2013) 385–407.
[10]J. Yong, F. Chen, M. Li, Q. Yang, Y. Fang, J. Huo, X. Hou, Remarkably simple achievement of superhydrophobicity, superhydrophilicity, underwater superoleophobicity, underwater superoleophilicity, underwater superaerophobicity, and underwater superaerophilicity on femtosecond laser ablated PDMS surfaces, J. Mater. Chem. A. 5 (2017) 25249–25257.
[11]M. Sanz, M. Oujja, C. Ascaso, S. P´erez-Ortega, V. Souza-Egipsy, R. Fort, A. de los Rios, J. Wierzchos, M.V. Ca˜namares, M. Castillejo, Influence of wavelength on the laser removal of lichens colonizing heritage stone, Appl. Surf. Sci. 399 (2017) 758–768.
[12]J.S. Pozo-Antonio, T. Rivas, M.E.L. de Silanes, A. Ramil, A.J. L´opez, Dual combination of cleaning methods (scalpel, biocide, laser) to enhance lichen removal from granite, Int. Biodeterior. Biodegrad. 168 (2022), 105373.
[13]A.J. L´opez, J.S. Pozo-Antonio, A. Moreno, T. Rivas, D. Pereira, A. Ramil, Femtosecond laser texturing as a tool to increase the hydrophobicity of ornamental stone: the influence of lithology and texture, J. Build. Eng. 282 (2022), 104176.
[14]J.F. Illescas, M.J.M.J. Mosquera, Producing surfactant-synthesized nanomaterials in situ on a building substrate, without volatile organic compounds, ACS Appl.
Mater. Interfaces 4 (2012) 4259–4269.
[15]M.J. Mosquera, L.A.M. Carrascosa, N. Badreldin, Producing superhydrophobic/oleophobic coatings on Cultural Heritage building materials, Pure Appl. Chem.
90 (2017) 551–562.
[16]D.S. Facio, M. Luna, M.J. Mosquera, Facile preparation of mesoporous silica monoliths by an inverse micelle mechanism, Microporous Mesoporous Mater. 247 (2017) 166–176.
[17]X. Jing, Z. Guo, Biomimetic super durable and stable surfaces with superhydrophobicity, J. Mater. Chem. A. 6 (2018) 16731–16768.
[18]L. Feng, Y. Zhang, J. Xi, Y. Zhu, N. Wang, F. Xia, L. Jiang, Petal effect: a superhydrophobic state with high adhesive force, Langmuir 24 (2008) 4114–4119.
[19]D.S. Facio, M.J. Mosquera, Simple strategy for producing superhydrophobic nanocomposite coatings in situ on a building substrate, ACS Appl. Mater. Interfaces 5 (2013) 7517–7526.
[20]D.S. Facio, L.A.M. Carrascosa, M.J. Mosquera, Producing lasting amphiphobic building surfaces with self-cleaning properties, Nanotechnology 28 (2017), 265601.
[21]F.G. Adamopoulos, E.C. Vouvoudi, E. Pavlidou, D.S. Achilias, I. Karapanagiotis, TEOS-based superhydrophobic coating for the protection of stone-built cultural heritage, Coatings 11 (2021) 135.
[22]W. Liu, L. Zhuang, J. Liu, Y. Liu, L. Wang, Y. He, G. Yang, F. Shen, X. Zhang, Y. Zhang, Make the building walls always clean : a durable and anti-bioadhesive diatomaceous earth @ SiO 2 coating, Construct. Build. Mater. 301 (2021), 124293.
[23]E. Bormashenko, Progress in understanding wetting transitions on rough surfaces, Adv. Colloid Interface Sci. 222 (2015) 92–103.
[24]A. Lafuma, D. Qu´er´e, Superhydrophobic states, Nat. Mater. 2 (2003) 457–460.
[25]T. Nishino, M. Meguro, K. Nakamae, M. Matsushita, Y. Ueda, The lowest surface free energy based on − CF3 alignment, Langmuir 15 (1999) 4321–4323.
[26]J. Yong, F. Chen, Q. Yang, D. Zhang, H. Bian, G. Du, J. Si, X. Meng, X. Hou, Controllable adhesive superhydrophobic surfaces based on PDMS microwell arrays, Langmuir 29 (2013) 3274–3279.
[27]M. Grehn, T. Seuthe, M. H¨ofner, N. Griga, C. Theiss, A. Mermillod-Blondin, M. Eberstein, H. Eichler, J. Bonse, Femtosecond-laser induced ablation of silicate glasses and the intrinsic dissociation energy, Opt. Mater. Express 4 (2014) 689.
[28]F. Chen, D. Zhang, Q. Yang, J. Yong, G. Du, J. Si, F. Yun, X. Hou, Bioinspired wetting surface via laser microfabrication, ACS Appl. Mater. Interfaces 5 (2013) 6777–6792.
[29]X. Li, L. Chang, R. Qiu, C. Wen, Z. Li, S. Hu, Microstructuring and doping of silicon with nanosecond laser pulses, Appl. Surf. Sci. 258 (2012) 8002–8007.
[30]J. Li, Y. Zhou, F. Fan, F. Du, H. Yu, Controlling surface wettability and adhesive properties by laser marking approach, Opt Laser. Technol. 115 (2019) 160–165.
[31]A.V. Kabashin, P. Delaporte, A. Pereira, D. Grojo, R. Torres, T. Sarnet, M. Sentis, Nanofabrication with pulsed lasers, Nanoscale Res. Lett. 5 (2010) 454–463.
[32]T.O. Yoon, H.J. Shin, S.C. Jeoung, Y.-I. Park, Formation of superhydrophobic poly(dimethysiloxane) by ultrafast laser-induced surface modification, Opt Express 16 (2008), 12715.
[33]B. Farshchian, J.R. Gatabi, S.M. Bernick, S. Park, G.-H. Lee, R. Droopad, N. Kim, Laser-induced superhydrophobic grid patterns on PDMS for droplet arrays formation, Appl. Surf. Sci. 396 (2017) 359–365.
[34]C. Yao, S. Xu, X. Jiang, J. Chen, X. Yuan, A simple way to achieve self-cleaning surfaces with unique antifouling property, J. Chem. 2020 (2020).
[35]L. Pinho, M.J. Mosquera, Titania-silica nanocomposite photocatalysts with application in stone self-cleaning, J. Phys. Chem. C 115 (2011) 22851–22862.
