Control of Surface Wettability via Strain Engineering

TL;DR

This study demonstrates that applying reversible biaxial strains to graphene can effectively and predictably control its surface wettability, enabling tunable applications in microfluidic and optical systems.

Contribution

The paper introduces a molecular dynamics-based approach and an analytic model showing how strain engineering can reversibly modulate surface wettability through changes in interfacial energy.

Findings

Contact angles vary from 72.5° to 106° under strains from -10% to 10%.

A linear relation exists between cos(θ) and applied strain.

The analytic model accurately predicts wettability changes based on adsorption energy.

Abstract

Reversible control of surface wettability has wide applications in lab-on-chip systems, tunable optical lenses, and microfluidic tools. Using a graphene sheet as a sample material and molecular dynamic (MD) simulations, we demonstrate that strain engineering can serve as an effective way to control the surface wettability. The contact angles $\theta$ of water droplets on a graphene vary from 72.5$^\circ$ to 106$^\circ$ under biaxial strains ranging from -10% to 10% that are applied on the graphene layer. For an intrinsic hydrophilic surface (at zero strain), the variation of $\theta$ upon the applied strains is more sensitive, i.e., from 0$^\circ$ to 74.8$^\circ$. Overall the cosines of the contact angles exhibit a linear relation with respect to the strains. In light of the inherent dependence of the contact angle on liquid-solid interfacial energy, we develop an analytic model to show the $\cos \theta$ as a linear function of the adsorption energy $E_{ads}$ of a single water molecule over the substrate surface. This model agrees with our MD results very well. Together with the linear dependence of $E_{ads}$ on biaxial strains, we can thus understand the effect of strains on the surface wettability. Thanks to the ease of reversibly applying mechanical strains in micro/nano-electromechanical systems (MEMS/NEMS), we believe that strain engineering can be a promising means to achieve the reversibly control of surface wettability.