Ashis Kumar Sen

Patterned superhydrophilic-superhydrophobic (SHL – SHB) surfaces have shown promise in droplet-based biochemical assays. However, fundamental understanding of the behavior of liquid droplets on such patterned surfaces has not received much attention. Here, we report wetting dynamics and stability of an aqueous droplet placed over a superhydrophilic spot (θ~0°) surrounded by a superhydrophobic surface (θ~160°). We study the shape evolution (contact angle (θ) and contact line diameter (dc)) of an aqueous droplet placed over a horizontal SHL – SHB surface with its volume (Vd), using experiments and analytical modeling. The results showed that depending upon the Bond number (Bo) and spot diameter (ds), three different regimes: spherical cap with fixed dc and varying θ (Regime I), oblate spheroid with fixed dc and varying θ (Regime II), and oblate spheroid with varying dc and fixed θ (Regime III), are observed. The transition from Regime I to Regime II occurs for Bo~1 whereas that from Regime II to Regime III occurs at Bocr~0.33ds1.30. Analysis of the present case wherein the contact line lies at the boundary of SHL – SHB surfaces, revealed anomaly with respect to the statements of Wenzel, Cassie-Baxter and McCarthy. Further, the stability of a droplet placed over the superhydrophilic spot on an SHL – SHB angular surface is studied using experiments and analytical modeling, which showed that the competition between contact line pinning force (Fp) and gravitational force (Fg) governs its stability. The stable and unstable regimes are identified based on the Bond number (Bo) and spot diameter (ds) and the critical Bond number for stable – unstable transition depends on spot diameter as Bocr~0.5ds-0.93.

Hypothesis: Surface acoustic waves (SAW) propagating along a solid surface can significantly affect the dynamics of droplet impact. Although droplet impact in presence of SAW has been attempted recently, here, we investigate the effects of surface wettability, droplet size, impact velocity, and SAW power on the impact and spreading dynamics along with post-impact oscillation dynamics of a drop. Experiments: Here, we study droplet impact on a surface exposed to traveling SAW produced using an interdigitated electrode patterned on a piezoelectric substrate. The effects of Weber number (We), surface wettability, and SAW power on the impact and spreading dynamics and post-impact oscillation dynamics are studied. Findings: Our study unravels that the interplay between capillary and viscous forces, and inertia forces arising due to pre-impact kinetic energy and SAW-induced bulk acoustic streaming underpins the phenomena. Remarkably, we find that the effect of SAW on droplet impact dynamics is predominant in the case of a hydrophilic (HPL) substrate at a higher SAW power and smaller We and hydrophobic (HPB) substrate irrespective of SAW power. Our study reveals that the maximum droplet spreading diameter increases with SAW power at smaller We for an HPL surface whereas it is independent of SAW power at higher We. Post-impact oscillation of a droplet over an HPL surface is found to be overdamped with a smaller amplitude compared to an HPB substrate, and a faster decay in oscillation amplitude is observed in the case of an HPB surface and higher We. Our study provides an improved understanding of droplet impact on a surface exposed to SAW that may find relevance in various practical applications.

The autonomous and directional transport of microdroplets is critical to the development of self-powered droplet-based microfluidic devices. So far the studies on the self-transport of droplets have employed chemically inhomogeneous surfaces. Remarkably, we observed the self-propulsion of aqueous droplets on a chemically homogeneous superhydrophilic surface. The self-transport phenomenon is attributed to the existence of a precursor layer between droplet and surface that minimizes contact line pinning and droplet is driven by a net force owing to the asymmetry in the precursor layer width across droplet. We provide a theoretical model considering the electrostatic, capillary, and evaporation induced forces to quantitatively predict transport velocity which is in good agreement with experimental observations. The effect of droplet volume, surface roughness, relative humidity, and composition on the droplet velocity is studied. We demonstrated the directional self-transport and coalescence of droplets on a superhydrophilic track in a superhydrophobic background and the self-transport of droplets containing biological cells depicting biological cargos.