Abstract
We use large-scale molecular dynamics to study dynamics at the three-phase contact line in electrowetting of water and electrolytes on no-slip substrates. Under the applied electrostatic potential the line friction at the contact line is diminished. The effect is consistent for droplets of different sizes as well as for both pure water and electrolyte solution droplets. We analyze the electric field at the contact line to show how it assists ions and dipolar molecules to advance the contact line. Without an electric field, the interaction between a substrate and a liquid has a very short range, mostly affecting the bottom, immobilized layer of liquid molecules which leads to high friction since mobile molecules are not pulled towards the surface. In electrowetting, the electric field attracts charged and polar molecules over a longer range which diminishes the friction.
Highlights
Recent developments in the study of liquid droplets spreading on surfaces have shown that the dynamics can be limited by a mixture of inertia and viscous and contact line energy dissipation
In this paper we investigate how electrowetting affects contact line friction on a molecular level in the rapid wetting regime, using computer molecular dynamics simulations of pure water and an electrolyte solution
Contact line friction is measured by how much the contact line speed v is damped compared to what we would expect from the Young driving force [10,23,24]
Summary
Recent developments in the study of liquid droplets spreading on surfaces have shown that the dynamics can be limited by a mixture of inertia and viscous and contact line energy dissipation. The term that dominates the process can be determined from the balance of nondimensional Ohnesorge numbers, wh√ich relate viscous fricti√on to surface tension and inertial forces [1]. These are given by Oh ≡ μ/ ργ R and Ohf ≡ μf/ ργ R, where μ and ρ are the liquid viscosity and density, respectively, γ is the liquid-vapor surface tension, R is the initial droplet radius, and μf is a contact line friction parameter which has units of viscosity. This is the case for certain hydrogen bonding [2,3] or microstructured substrates [4]
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