Electrowetting Actuation of Polydisperse Nanofluid Droplets

Abstract

We present results of electrowetting experiments employing droplets formed from aqueous suspensions of Au nanoparticles. A planar electrowetting system, consisting of a Pt wire electrode and a bottom Cu electrode with an insulating silicone layer, is used to observe changes in droplet contact angle when an external electric field is applied. The equilibrium contact angle at 0 V decreases with increasing nanoparticle concentration, dropping from 100.4° for pure deionized water to 94.7° for a 0.5 μM nanofluid. Increasing the nanoparticle content also lowers the required voltage for effective actuation. With actuation at 15 V, contact angle decreases by 9% and 35% for droplets formed from pure water and a 0.5 μM nanoparticle suspension, respectively. Contact angle saturation is observed with nanofluid droplets, with the threshold voltage decreasing as nanoparticle concentration rises. Maximum droplet actuation before contact angle saturation is achieved at only 10 V for a concentration of 0.5 μM. A proposed mechanism for the enhanced electrowetting response of a nanofluid droplet involves a reduction in surface tension of the droplet as nanoparticles accumulate at the liquid-vapor interface.

Electrowetting-on-dielectric behavior of aqueous droplets and gold nanofluid on an electrospun poly(vinyl chloride) microfiber layer

“…Effective liquid-gas surface tensions are calculated to be 0.070 N m −1 , 0.064 N m −1 , and 0.066 N/m for water, KOH and KCl, respectively. We note that the g lg value obtained for water is consistent with the actual g lg of water at room temperature [69][70][71][72]. The g lg for the nanofluid was determined to be markedly different from that of the aqueous solutions, with a calculated value of 0.025 N m −1 .…”

“…The g lg for the nanofluid was determined to be markedly different from that of the aqueous solutions, with a calculated value of 0.025 N m −1 . This surface tension falls within the range of g lg reported for nanofluids containing quantum dots [73] and gold nanoparticles [69]. Using this effective g lg in the Young-Lippman equation, better agreement is achieved between the theoretical curve and experimental data for the effect of applied voltage on CA (figure 7(b)), specifically at low voltages.…”

“…Figure 8(a) illustrates the suspended Au nanoparticles in the droplet. In the work of Patacsil et al [69] and Kumar et al [70], the nanoparticles (figure 8(a)) were considered as polar dielectrics that were suspended in the liquid medium. When a potential difference is applied, nanoparticles are polarized, creating a series of capacitors within the droplet as shown schematically in figure 8(b).…”

“…This repulsive interaction among the nanoparticles reduces the intermolecular attraction of water molecules, thereby decreasing the water-water attraction and liquid-gas surface tension [73], as illustrated in figure 8(d). It is well-known that the inherent high surface area to volume ratio of nanoparticles actively affects the behavior of the fluid in which they are suspended [69][70][71][72][73].…”