Effect of substrate topography, material wettability and dielectric thickness on reversible electrowetting

Abstract

Recent experiments by Kavousanakis et al., Langmuir, 2018 [1], showed that reversible electrowetting on superhydrophobic surfaces can be achieved by using a thick solid dielectric layer (e.g. tens of micrometers). It has also been shown, through equilibrium (static) computations, that when the dielectric layer is thick enough the electrostatic pressure is smoothly distributed along the droplet surface, thus the irreversible Cassie to Wenzel wetting transitions can be prevented. In the present work we perform more realistic, dynamic simulations of the electrostatically-induced spreading on superhydrophobic surfaces. To this end, we employ an efficient numerical scheme which enables us to fully take into account the topography of the solid substrate. We investigate in detail the role of the various characteristics of the substrate (i.e. the dielectric thickness, geometry and material wettability) and present relevant flow maps for the resulting wetting states. Through our dynamic simulations, we identify the conditions under which it is possible to achieve reversible electrowetting. We have found that not only the collapse (Cassie-Baxter to Wenzel) transitions but also the contact angle hysteresis of the substrate significantly affects the reversibility.

Highlighting the Role of Dielectric Thickness and Surface Topography on Electrospreading Dynamics

“…In particular, as thoroughly presented in [14,15,16], the liquid-solid interactions are lumped in an effective pressure term, pLS, which will now be accounted in the normal component of the interface force balance:τnn|liquid=sans-serifΔp−γLA0.166667emC−pLS−pel, where C is the local mean curvature, sans-serifΔp is the pressure jump across the interface, γLA is the liquid-ambient interfacial tension, pel is the electrostatic pressure generated as a result of the electric field. We note that since the liquid is considered as perfectly conducting (assuming a high electrolyte concentration), the electric field at the liquid surface is normal to the surface, and such is also the direction of the electrostatic pressure.…”

“…The above expression, introduced in [14,15,16], essentially approximates a Lennard-Jones type potential where R0 is the initial droplet radius at t = 0. Apart from the latter expression, other possible formulations for the disjoining pressure (for example using an exponential function) could also be employed, as demonstrated in [17].…”

“…In our study, we have neglected any gravitational force in the Navier-Stokes equations. The tedious task of modeling multiple three-phase contact lines, on the structured surface, has been overcome by employing a sharp-interface scheme which has proven to be very efficient for the study of droplet’s dynamic behavior in our previous works [ 14 , 15 , 16 ]. According to this scheme, the liquid-vapor and the liquid-solid interfaces of the droplet are treated in a unified manner.…”

“…As observed in the inset, a Wenzel state is observed at the arbitrary-structured topography ( a ) whereas a Cassie-Baxter state is accommodated in stripe-structured topography ( b ). The disjoining pressure parameters we use are, according our previous work (see [14,15,16]): C1 = 12, C2 = 10, σ = 9 × 10 −3 and ϵ = 8 × 10 −3 (resulting in δmin = 1.25 μm) while the dimensionless slip parameter: βLS = 10 3 .…”

“…In particular, we use a sharp-interface model, that has been developed by our team, where both the liquid-vapor and the liquid-solid interfaces are treated in a unified manner. Here we employ the notion of the Derjaguin (disjoining) pressure to model the micro-scale liquid-solid interactions [ 14 , 15 , 16 ]. In this work we investigate different substrate cases by varying the surface topography and the dielectric thickness.…”