Session G26: Drops: Superhydrophobic Surfaces II

Droplet manipulation to detect surface tension

Recent years have witnessed a significant spike in manipulation of liquid droplets because of their applications in microfluidic diagnostics, drug discovery etc. Particularly, droplet manipulation on super-repellent surfaces has been widely studied using various methods. However, to the best of our knowledge, there are no studies that employ super-repellent surfaces to sort droplets based on surface tension. In this work, we synthesized tunable superomniphobic surfaces with fluorinated, flower-like TiO$_{\mathrm{2}}$ nanostructures. We demonstrate that the surface chemistry, and consequently the solid surface energy of our superomniphobic surfaces can be tuned using UV irradiation. This allows us to systematically tune the mobility of droplets with different surface tensions on our superomniphobic surfaces. Each of these surfaces with same surface texture, but different solid surface energy allows certain high surface tension liquid droplets to freely roll past the surface while ``trapping'' other low surface tension liquid droplets. Leveraging this selective mobility of droplets based on their surface tension, we fabricated a simple device with precisely tailored discrete surface energy domains that can sort droplets by their surface tension. We envision that our droplet sorting device will enable inexpensive analytical devices for personalized point-of-care diagnostic platforms.   

Droplet sliding on inclined superhydrophobic surfaces: the effect of anisotropic contact line.

Although the effects of solid structures on droplet retention on superhydrophobic surfaces have been studied extensively, the investigation has been restricted to the sessile droplets on horizontal surfaces where the contact line motions are axisymmetric or isotropic (either advancing or receding). In the droplet retention on inclined surfaces, the contact line motions are asymmetric or anisotropic; the advancing and receding motions coexist. In this study, we investigate the correlation between the droplet boundary pinning and the surface morphology on inclined superhydrophobic surfaces. The evolution of the droplet contact angle and width show contrary behaviors between pillar- and pore-structured surfaces due to the distinctive microscopic contact line motions. Therefore, the visualizations of the contact line motions at different locations of the boundary on inclined superhydrophobic surfaces are performed and the averaged contact line density of the boundary is quantified. The result shows that the droplet retentive force monotonously increase with the increase in contact line density, regardless of the surface morphological types, dimensions, or the direction of contact line motion (advancing, receding, or both). The result indicates that the droplet retentive force on superhydrophobic surfaces is mainly determined by the contact line density, regardless of the isotropy of the contact line.   

Freezing-driven droplet self-removal from nanoengineered surfaces

Wetting and surface icing are important in nature and technical applications. To design surfaces capable of passively repelling water and ice, further research at the intersection of interfacial thermofluidics, nucleation thermodynamics, and materials engineering is required. We show and explain a previously unexplored freezing-driven droplet self-removal mechanism. We find that a sessile droplet being cooled mainly from its free surface can experience non-simultaneous freezing starting at the droplet free surface, while the droplet-substrate contact remains mostly liquid and freezes last. The ice-liquid phase boundary motion, which proceeds radially inward during freezing, coincides with an inward motion of the vapor-liquid-substrate contact line and can lead to complete dewetting. Concurrently, the confinement and inward growth of the solid ice shell, combined with the volumetric expansion due to the phase change, the incompressibility of ice and liquid, as well as the soft, unfrozen droplet-substrate interface result in the unfrozen core to be displaced towards the substrate causing droplet lifting and removal. We demonstrate the passive, self-cleaning mechanism on a variety of substrates, ranging from smooth and hydrophilic to nanoengineered and superhydrophobic.   

Imparting Icephobicity with Substrate Flexibility

Ice accumulation poses serious safety and performance issues for modern infrastructure. Rationally designed superhydrophobic surfaces have demonstrated potential as a passive means to mitigate ice accretion; however, further studies on solutions that reduce impalement and contact time for impacting supercooled droplets are urgently needed. Here we demonstrate the collaborative effect of substrate flexibility and surface texture on enhancing icephobicity and repelling viscous droplets. We first investigate the influence of increased viscosity on impalement resistance and droplet--substrate contact time. Then we examine the effect of droplet partial solidification on recoil by impacting supercooled water droplets onto surfaces containing ice nucleation promoters. We demonstrate a passive method for shedding partially solidified droplets that does not rely on the classic recoil mechanism. Using an energy-based model, we identify a previously unexplored mechanism whereby the substrate oscillation governs the rebound process by efficiently absorbing the droplet kinetic energy and rectifying it back, allowing for droplet recoil. This mechanism applies for a range of droplet viscosities and ice slurries, which do not rebound from rigid superhydrophobic substrates.   

Study on bouncing motion of a water drop collision on superhydrophobic surface under icing conditions

When micro droplets in the air are supercooled and collide with the object, they froze on the surface at the time of a collision and can be defined as icing. If supercooled water droplets collide with an airfoil of an aircraft in flight and shape changes, there is a danger of losing lift and falling. Recently, the ice protection system using a heater and Anti- / De-icing (superhydrophobic) coating is focused. In this system, colliding water droplets are melted by the heat of the heater at the tip of the blade, and the water droplet is bounced by the aerodynamic force on the rear superhydrophobic coating. Thus, it prevents the phenomenon of icing again at the back of the wing (runback ice). Therefore, it is possible to suppress power consumption of the electric heater. In that system, it is important to withdraw water droplets at an extremely superhydrophobic surface at an early stage. However, research on bouncing phenomenon on superhydrophobic surface under icing conditions are not done much now. Therefore, in our research, we focus on one drop supercooled water droplet that collides with the superhydrophobic surface in the icing phenomenon, and aim to follow that phenomenon. In this report, the contact time is defined as the time from collision of a water droplet to bouncing from the superhydrophobic surface, and various parameters (temperature, speed, and diameter) on water droplets under icing conditions are set as the water drop bouncing time (contact time) of the product.   

Effective slip lengths for immobilized superhydrophobic surfaces

Analytical solutions are found for both longitudinal and transverse shear flow, at zero Reynolds number, over immobilized superhydrophobic surfaces comprising a periodic array of near-circular menisci penetrating into a no-slip surface and where the menisci are no longer shear-free but are taken to be no-slip zones. Explicit formulas for the associated longitudinal and transverse effective slip lengths are derived; these are then compared with analogous results for superhydrophobic surfaces of the same characteristic geometry but where the menisci are shear-free. The new formulas give results that are consistent with recent experimental observations that have prompted suggestions that menisci that are assumed to be free of shear have in fact been immobilized.   

Continuum Modeling of a Water Droplet sitting on a Vibrating Superhydrophobic Surface

Because of the complex, multiscale nature, modeling of droplet-surface interaction remains a challenge. To understand the underlying mechanisms is important for application design. The interactions among liquid-gas-solid molecules dominate the contact line dynamics, and determines the stationary and dynamic contact angles. We propose a novel numerical method to handle the droplet on a superhydrophobic surface, and validate our model with experiments on a 3mm water droplet sitting on a vibrating surface. Different cases have been investigated for validating our methods and understanding of the vibration mechanism of droplet shedding. Although the vibration-induced wetting transition was investigated in recent studies, the vibration mechanism of droplet shedding has not yet been fully understood. This research quantitatively considers the effect of vibration on droplet shedding under various vibration resonance conditions, providing a possible way to effectively shed droplet off surfaces in condensation applications.   

Modeling thermal transport to droplets impinging on horizontal superhydrophobic surfaces

An analytical model is developed to quantify the heat transfer to droplets impinging on heated superhydrophobic surfaces. Integral analysis is used to incorporate the apparent temperature jump at the superhydrophobic surface as a boundary condition. This model is combined with a fluid model which incorporates velocity slip to calculate the cooling effectiveness, a metric outlined in contemporary work. The effect of varying velocity slip and temperature jump is analyzed for different impact Weber numbers and surface temperature ranging from 60 to 100 $^{\circ}$C. Heat transfer to the drop on superhydrophobic surfaces is decreased when compared to conventional surfaces.