Anti-gravity drop motion and co-solute transport of hexamethyldisilane - tetrahydrofuran binary mixture on glass surface

Abstract

Surface wettability modulation have garnered significant research interest due to diverse applications across fields ranging from microfluidics to biomedical devices. Surface wettability determined by factors like chemical composition and surface topography, influences a surface’s ability to attract or repel liquids. An essential phenomenon within surface wettability is the ability of droplets to move against gravity, known as uphill motion. This motion is particularly important in microfluidic devices requiring precise droplet control. Creating a surface with tuneable wettability supporting uphill motion is critical for optimizing device design. In this work, we proposed a simple and effective way to achieve tetrahydrofuran droplet self-running horizontally and sliding uphill without any other external force. We have used a complete wetting glass surface onto which binary non-aqueous THF drop containing hexamethyldisilane (HMDS) demonstrated uphill drop motion with tilted angle of ∼ 10°. This binary drop mixture of volatile THF and HMDS with various concentrations demonstrated self-propulsion on both the horizontal and tilted surfaces. We also examined the transport of an additional solute utilizing the binary mixture of HMDS-THF. The droplet motion against the gravity was achieved by gradient in solute concentration-based Marangoni stress. The potential applications of droplet motion against gravity on tilted surfaces span various domains. It can be harnessed for self-cleaning surfaces, microfluidic designs, and controlled liquid transport systems.