Drop Capturing Based on Patterned Substrate in Space

Enhancement of Meniscus Pump by Multiple Particles

We numerically investigate the behavior of a droplet spreading on a smooth substrate with multiple obstacles. As experimental works have indicated, the macroscopic contact line or the three-phase boundary line of a droplet exhibits significant deformation resulting in a local acceleration by successive interactions with an array of tiny obstacles settled on the substrate (Mu et al., Langmuir 2019, 35). We focus on the menisci formation and the resultant pressure and velocity fields inside a liquid film in a two-spherical-particle system to realize an optimal design for the effective liquid-transport phenomenon. Special attention is paid to the meniscus formation around the second particle, which influences the liquid supply related to the pressure difference around the first particle as a function of the distance between the two particles. We find that the meniscus around the first particle plays an additional role as the reservoir of the liquid supplied toward the second particle, which is found to enhance the total pumping effect.

Evaporation of Sessile Water Droplets on Horizontal and Vertical Biphobic Patterned Surfaces

This paper presents an experimental study on thermal transport to single water droplets evaporating on heated biphobic surfaces consisting of a superhydrophobic matrix with a circular hydrophobic pattern with strong contact line pinning. A single water droplet of 8 μL volume is placed on a preheated surface and allowed to evaporate in an open laboratory environment. We investigate the influence of substrate orientation (horizontal and vertical) on evaporation dynamics. Using optical and infrared imaging, we report droplet fluid dynamics and heat transfer characteristics of the evaporating droplet. Overall, evaporation is more efficient on the vertical surface, exhibiting higher total heat transfer rates and up to 10% shorter evaporation times. Counterintuitively, on the vertical surface, the substrate-droplet interfacial heat flux was higher near the lower contact line than in the upper region, despite a high contact angle and an expected wedge effect at the bottom. At the same time, the temperature is colder in the lower part of the droplet. We attribute this apparent anomaly to the competition between sensible heating and evaporation, and a modified convective flow signature (both within the droplet and the gas phase) compared to a horizontal surface. We also show that the thermal signature becomes uniform once the contact angles at the upper and lower contact lines become equal toward the end of the evaporation process. Insights from this work can guide the design of spray cooling devices or be used to alter particle deposition patterns during evaporation-based fabrication techniques and ink-jet printing.

Pattern Formation in Drying Sessile and Pendant Droplet: Interactions of Gravity Settling, Interface Shrinkage, and Capillary Flow

We reported the interactions of the gravitational sedimentation, interface shrinkage, and outward capillary flow in drying droplets. This coupling effect is the inference we draw from deposition patterns of both sessile and pendant droplets, which contain particles of different sizes, evaporating on a patterned substrate. The deposition difference between sessile and pendant droplets containing microparticles indicated that gravitational sedimentation has a significant influence on the deposition morphology. The phase diagram shows that the particle deposition process can be divided into two stages: in the first stage, the competition between the interface shrinkage and the gravitational sedimentation determines whether the particles can be captured by the liquid-air interface; in the second stage, the capillary flow takes the particles inside the droplet toward the edge. The deposition morphology is the result of competition and cooperation interactions of the free setting, interface shrinkage, and outward capillary flow.