Caracterización

The schematic diagram of experimental setup for evaluating the studied TSE

Systems twin-head electrospray (THES) disperser of nanoparticles is shown in Figure 4.1. The THES case is a metal 6-way cross (i.e., formed by three straight cylindrical channels

perpendicular to and intersecting with each other), thus having six openings. The top and the

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of particle-laden flow, respectively. The volumetric flow rate of primary carrier air is

variable by a needle valve and monitored by a flow meter. The front and the back openings are used as windows to illuminate the spray chamber by a light source and to observe the

spray process by a microscopic lens and a CDD camera. The left- and right-hand side

openings of the spray chamber are used to install two identical electrospray capillaries,

aligned in the axial direction and faced against each other. The tested stainless steel

capillaries have an inner diameter (ID) of 0.7 mm and an outside diameter (OD) of 1.0 mm.

A sheath flow compartment is also included in the installation of each capillary to provide

clean air flow sheathing the capillary. To operate the THES disperser, a positive DC high

voltage was applied on one capillary and a negative voltage on the other (by the H.V. power

supplies: Bertan Series 230, Spellman High Voltage Electronics Corporation, Valhalla, NY).

The spray chamber was electrically grounded. The spray suspension was fed into two

capillary needles by two identical syringe pumps (NE-300, New Era Pump System Inc.

Farmingdale, NY). For simplicity in operation, the feeding flow rates of two capillaries were

set the same. The flow rate of capillary sheath gases for two capillaries were also kept the

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Figure 4.1: Schematic diagram of the studied twin-head electrospray disperser and its experimental setup for performance evaluation.

Distinguished from the typical cone-jet mode electrospray (applied in the majority

of literature), the THES disperser operated the spray at the stable multiple-jet mode to produce a stream of particles in high aerosol concentration. The sizes of nanoparticles to be

tested are pre-determined by their synthesis. There was no concern about altering the sizes

of nanomaterials. The multiple-jet electrospray used in the disperser is just a means to

disperse nanoparticles in air with the minimal agglomeration. Notice that the stable multi-

jet electrospray described herein was operated at the mode when the maximal number of stable jets at the capillary tip was observed by a CCD camera with a microscopic lens.

To investigate the performance of the studied THES disperser, a Scanning Mobility

Particle Sizer (SMPS) (including TSI 3080 electrostatic classifier and TSI 3025 UCPC, TSI

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dispersed nanoparticles. During the operation, a camera with a microscopic lens was set up

to observe and record the liquid meniscuses at the capillary tips.

To characterize the mass concentration of dispersed nanoparticles in the carrier gas

stream output from the THES disperser, a stainless steel filter holder with a pre-weighted

HEPA filter (Whatman 934-AHT M_{, 47 mm φ, GE Healthcare Bio-Sciences Corporation,}

Psicataway, NJ) was installed at the downstream of the THES to collect all the particles in

the gas stream exiting the system. The filter after collecting particles was kept in well-

ventilated lab environment (usually kept at 30% RH) for more than three hours prior to its

mass measurement to minimize the solvent contribution. The mass of particles collected on

the HEPA filter was calculated as the filter mass difference before and after the collection.

The total particle concentration in the exiting gas stream was derived as follows:

𝐶 (𝑚𝑔 𝑚3) =

𝑀_{𝑙𝑜𝑎𝑑}−𝑀_{𝑐𝑙𝑒𝑎𝑛}

(𝑄𝑀+2∗𝑄𝑆)𝑡 (4-1)

where 𝑀_{𝑐𝑙𝑒𝑎𝑛} and 𝑀_{𝑙𝑜𝑎𝑑} are the filter masses before and after particle collection, respectively; 𝑄_𝑀 and 𝑄_𝑆 are the flow rates of primary carrier and capillary sheath flows, respectively; t is the particle collection time in each run.

To achieve high mass concentration of particles in the gas stream, the following

parameters were varied during the study: needle tip distance, main carrier flow rate, side

sheath flow rate, liquid feeding rate and nanoparticle concentration in spray suspensions.

Three different nanomaterials, i.e., TiO2 (99.7%, <25nm, Sigma-Aldrich Co.), ZnO

(99.9+%, 20 nm, Nanostructured & Amorphous Materials Inc.) and NiO (20 nm,

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three most popular nanoparticles reported in toxicity studies and applications. Previous

research showed that the breakage induced by nano TiO2 and the oxidative stress under the exposure of nano TiO2 could cause adverse effects on erythrocyte, implying a potential

toxicity to human health (Li et al., 2008). Brunner et al. (2006) found that nano ZnO with

the concentration above 15 ppm could kill almost all human or rodent cells. NiO was

classified as a Class 1 carcinogenic material by the International Agency for Research on

Cancer (IARC; IARC Monographs on the Evaluation of Carcinogenic Risk to Humans,

Volume 49 Chromium, Nickel and Welding 1990; cited in Horie et al., 2012). In this study,

spray liquids were prepared by dispersing nanoparticles (i.e., TiO2, ZnO and NiO) in

isopropanol (IPA)/aqueous (1:1 in v/v) mixtures at various concentrations ranging from 1.0

g/L to 10.0 g/L. A trace amount of nitric acid was added into the nanoparticle suspensions

to control the electrical conductivity of spray suspensions at approximately 150 µS/cm. To

keep the colloidal suspensions stable during the entire experimental run, the spray

suspensions were sonicated by an ultrasonic processor (CPX 750, Cole-Parmer Instrument,

IL).