Electric field induced spreading of nanodroplet is simulated using molecular dynamics (MD) simulation to understand the physics of molecular motion near the three phase contact line (TPCL). Electric field assisted droplet movement has a wide range of application starting from electrochemical cells, membrane ion channels, to modification and characterisation of biological cells on Lab-on-a-chip based micro-fluidic devices. When electric field induced droplet moves on a surface it encounters various resistive forces and contact line friction force is found to be the dominating one, which controls the droplet movement. The origin of this contact line friction (CLF) force is molecular in nature and can be explained from the molecular movement near the TPCL. However, there is a dearth on the understanding of the CLF from its origin. In the present study, MD simulation has been performed on the spreading of a nanodroplet on a solid surface to unveil the CLF from its origin. In MD regime the solid surface is characterised by adjusting the depth of the potential well (ε) of liquid-solid Lennard-Jones interaction parameter. Our finding show, with variation in electric field (in the direction parallel to the surface), the droplet footprint and the contact angle (both leading & trailing) changes significantly. At 1V/nm electric field, 50% increase in the droplet footprint area and 32% decrease in contact angle is observed. A faster dynamic response of spreading in presence of electric field on a hydrophobic surface is observed. Movement of the TPCL during the spreading of the droplet is resisted by the CLF force which is manifested by TPCL friction coefficient ζ, which is a function of the temperature, equilibrium frequency (k<sup>0</sup>) and average molecular displacement (λ). The equilibrium frequency during the TPCL movement is affected by the temperature, substrate property or presence of any external disturbances, which lead to an alteration of ζ. It is observed that ζ reduces by 20% with 1V/nm electric field. Significant decrease in ζ (~39%) with only 20K increase in temperature is observed for electrowetted nanodroplet. The alteration of CLF with temperature will have a potential application towards the characterisation of surfaces for electric field induced nanodroplet spreading.

Molecular investigation of the three phase contact line friction of electrowetted nanodroplet

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.