TRATAMIENTO DE LA INFORMACIÓN

A flow focussing area on the main channel was made on P-2000 machine. The machine heats the capillary with laser pulses and simultaneously pulls it, which results in the formation of two pieces of micropipettes. However, if the cycle is interrupted the capillary stays unbroken with a bottleneck-like area. We used the following machine settings: heat = 550 °K, filament = 4, velocity = 10, delay = 121, pull = 12. The cycle was stopped after three pulses of the laser. Further, at the distance of 15 mm from the thinnest part of the flow focussing area a hole was made by gentle scratching. A 40 mm long standard capillary was attached to the hole by epoxy adhesive forming the connected side channel at 90° angle. Thereafter, a 32 gauge needle was inserted inside the main channel placing the tip in the centre of the bottleneck area and fixed by epoxy glue. Simultaneously, adhesive blocked the capillary from the edge of the needle penetration making it an exclusive continuation of the channel. After this, a short capillary was drilled and divided so that cut line was in the centre of the perforated hole. The protrusive end of the needle was inserted in one of these pieces and fixed by epoxy

glue forming a new separated segment of the main channel. Another 32 gauge needle was bent at 90° angle and fixed in the piece of capillary with one end, while the other was aligned with the first needle’s tip at a distance of 0.5 mm by means of the optical microscopy. Then the second part of the bisected capillary was connected with the first, seizing the needle inside, and forming the second orthogonal channel on the opposite side of the main channel, followed by sealing the hole with a fast curing epoxy glue. After these steps, the obtained device was attached to the microscopy glass slide in order to increase its structural strength (see Figure II-8).

Figure II-8. Schematic illustration of the manufacturing process of the simplified co- flow double emulsion droplet generator.

II.3 References

[1] Haddleton, D. M.; Crossman, M. C.; Dana, B. H.; Duncalf, D. J.; Heming, A. M.; Kukulj, D.; Shooter, A. J., Atom Transfer Polymerization of Methyl Methacrylate Mediated by Alkylpyridylmethanimine Type Ligands, Copper(I) Bromide, and Alkyl Halides in Hydrocarbon Solution. Macromolecules 1999, 32, 2110-2119.

[2] Wooding, A.; Kilner, M.; Lambrick, D. B., Studies of the double surfactant layer stabilization of water-based magnetic fluids. J. Colloid Interface Sci. 1991, 144, 236-242.

[3] Shen, L.; Laibinis, P. E.; Hatton, T. A., Bilayer Surfactant Stabilized Magnetic Fluids:  Synthesis and Interactions at Interfaces. Langmuir 1998, 15, 447-453. [4] Maity, D.; Agrawal, D. C., Synthesis of iron oxide nanoparticles under oxidizing

environment and their stabilization in aqueous and non-aqueous media. J. Magn.

Magn. Mater. 2007, 308, 46-55.

[5] Yin, Y.; Li, Z.-Y.; Zhong, Z.; Gates, B.; Xia, Y.; Venkateswaran, S., Synthesis and characterization of stable aqueous dispersions of silver nanoparticles through the Tollens process. J. Mater. Chem. 2002, 12, 522-527.

[6] Cahn, J. W., Critical point wetting. J. Chem. Phys. 1977, 66, 3667-3672.

[7] Robledo, A.; Varea, C.; Indekeu, J. O., Wetting transition for the contact line and Antonov’s rule for the line tension. Physical Review A 1992, 45, 2423-2427.

[8] Kruss GmbH, Measurement contact angle.

http://www.kruss.de/en/theory/measurements/contact-angle/measurement- contact-angle.html (accessed 15 December 2012).

[9] Quevedo, E.; Steinbacher, J.; McQuade, D. T., Interfacial Polymerization within a Simplified Microfluidic Device:  Capturing Capsules. J. Am. Chem. Soc. 2005, 127, 10498-10499.

[10] Song, J.-S.; Winnik, M. A., Cross-Linked, Monodisperse, Micron-Sized Polystyrene Particles by Two-Stage Dispersion Polymerization. Macromolecules

2005, 38, 8300-8307.

[11] Wang, D.; Dimonie, V. L.; Sudol, E. D.; El-Aasser, M. S., Seeded dispersion polymerization. J. Appl. Polym. Sci. 2002, 84, 2710-2720.

CHAPTER III

Synthesis of Anisotropically-shaped Amphiphilic

Microparticles

In this chapter we show that induced internal phase separation can transform the microfluidically generated droplets into multifunctional anisotropic polymer-based structures. An application of the microfluidic device gives a control over the size and monodispersity of synthesized objects. A blend of non-polymerisable oil with hydrophilic and hydrophobic monomers allows formation of amphiphilic "microbuckets" with an outer hydrophilic surface and an inner hydrophobic cavity. We provide experimental evidence of the surface amphiphilicity by exposing our "microbuckets" to selective hydrophilic-hydrophobic interactions. Finally we demonstrate that magnetic microbuckets can be used to capture oil droplets dispersed in water.

III.1 Introduction

Recent progress in synthetic routes towards anisotropic microparticles provided a wealth of opportunities to fabricate anisotropic materials of various patterns, shapes, compositions and functionalities.1 Where anisotropy is imparted through shape, the resulting colloids show radically different behaviour at interfaces.2,3 Where the chemical nature of the colloid is heterogeneous, particles have been demonstrated to work efficiently in electrophoretic display technologies,4 as bifunctional biological receptors5 and have the ability to self-assemble under particular conditions.6,7 Exploitation of both chemical and shape inhomogeneity allows for an immense range of structures with a variety of applications. Amphiphilic particles with a peanut type morphology for example, have the ability to self-assemble into supracolloidal structures similar to those formed by amphiphilic molecules, the assembly process driven by hydrophobic attractive forces and governed by a geometric packing parameter.8These biphasic type particles can be produced by a multitude of methods including seeded polymerization of monomer swollen crosslinked particles,9 continuous-flow lithography10 and by phase separation of emulsion droplets upon evaporation of a volatile solvent.11-15

One fabrication technique which has played, and continues to play, a prominent role in this revolution in synthetic methodology, is droplet based microfluidics. The versatility of microfluidics has allowed synthesis of diverse complex microstructures,16 for instance non-spherical particles,17-21 multiphase "Janus" particles,22-25 porous structures,26-32 giant polymer vesicles33-38 and microgels.39-42 Such diversity of the synthesized particles is achieved, in many respects, by virtue of the performance of microfluidic devices which can be very simple43 or complex setups,44, 45 requiring complex technologies, for instance lithography or glass capillary pulling. The finite

structure of the particles defines the type of the device which is employed in the process: simple single emulsion generators are suitable for non-hollow particles, while capsule-like microstructures demand utilization of more complex multiple emulsion units.

We were inspired by the possibilities to create intricate functional microstructures by combining the phenomenon of internal phase separation and microfluidic techniques. Taking as a basis of inspiration internal phase separation studies presented by Vincent et al. who synthesized oil core-polymer shell microcapsules for controlled release,46-48 and freezing/warming method of toluene swollen polystyrene beads which led to formation of "bucket-like" polymer colloids by Xia and coworkers,49 we set ourselves the goal to develop a method of fabrication of polymer "microbuckets" using induced phase separation as a tool to create hollow shapes. We used a simple approach to make monodisperse biphasic microparticles that does not rely heavily on complex microfluidic device geometry, but rather on interfacial tension of the selected fluids. We set a requirement that the outer surface of our microbuckets had to be hydrophilic to optimize wettability with water and the inner surface of its cavity had to be hydrophobic, so that the microbuckets would have the ability to take up and store oil. Our intention was to design and employ a microfluidic device which can be manufactured with affordable materials (see Figure II-3) which generates droplets of ~100 microns in diameter or less to be in range which microfluidics supposed to provide. To fabricate our microbuckets an emulsified droplet containing acrylic monomer and silicone oil is polymerized to induce phase separation, producing a solid- liquid Janus particle. The use of a hydrophilic, surface active monomer (2-hydroxyethyl methacrylate) imparted amphiphilic surface properties on the “microbucket” particle.