Electrowetting diminishes contact line friction in molecular wetting

We use large-scale molecular dynamics to study the 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.

Figure 1

Electrowetting system with a 15-nm-radius water droplet. (a) Two-dimensional slice of the system, highlighting the electrode charge. Below the substrate is a layer of countercharges. (b) System during an electrowetting experiment. (Figures were created using matplotlib [13].)

Figure 2

(a) Spreading base radius and (b) contact angle for all systems. The droplet radius is noted for the pure water systems. The applied potential sign is shown for all systems.

Figure 3

Measured mean contact line friction during the rapid spreading phase for all systems. Vertical bars mark $\pm 1$ standard error.

Figure 4

Radial distribution of measured dipole orientation inside droplets during (a) spontaneous wetting and (b) electrowetting of pure water. Insets show the averaged dipole direction and magnitude at contact lines where the arrows are scaled by a factor of 5 for the spontaneous view. The moment is normalized by the dipole moment of the water molecule.

Figure 5

Radial components of the dipole force ${\mathbf{F}}_p$ for water molecules at height $z$ above the top substrate atoms. Here $F_z$ is positive for a force directed towards the substrate.

Figure 6

The Young force $F_{\text{Y}}$ between the substrate and liquid mostly affects the bottom water layer (shaded red with thick outlines). The dipole force $F_p$ is shown as a longer range, reaching upper molecule layers (shaded blue).