Session A16: Drops: Condensation and Evaporation

Interaction of levitating microdroplets with moist air flow in the contact line region

Self-organization of levitating microdroplets of condensate over a liquid-gas interface has been observed in several recent experiments involving evaporation at high heat fluxes, although the nature of thin phenomenon is still not completely understood. We conduct experimental investigation of behavior of an ordered array of levitating microdroplets as it approaches a region of intense evaporation near the contact line. Interaction of the array with the local highly non-uniform gas flow is shown to result in the break-up of the pattern. Furthermore, our experimental set-up provides a unique tool for investigation of the Stefan flow originating near the contact line by using microdroplets as tracers. Local gas flow velocities near the contact line are obtained based on trajectories of the droplets.   

Analysis of water microdroplet condensation on silicon surfaces

We observed the condensation process of water microdroplets on flat silicon (100) surfaces by means of the sequential visualization of the droplets using an environmental scanning electron microscope. As previously reported for nanostructured surfaces, the condensation process of water microdroplets on the flat silicon surfaces also exhibits two modes: the constant base (CB) area mode and the constant contact angle (CCA) mode. In the CB mode, the contact angle increases with time while the base diameter is constant. Subsequently, in the CCA mode, the base diameter increases with time while the contact angle remains constant. The dropwise condensation model regulated by subcooling temperature does not reproduce the experimental results. Because the subcooling temperature is not constant in the case of a slow condensation rate, this model is not applicable to the condensation of the long time scale (\textasciitilde several tens of minutes). The contact angle of water microdroplets (\textasciitilde several $\mu $m) tended to be smaller than the macro contact angle. Two hypotheses are proposed as the cause of small contact angles: electrowetting and the coalescence of sub-$\mu $m water droplets.   

Condensation on Slippery Asymmetric Bumps

Controlling dropwise condensation by designing surfaces that enable droplets to grow rapidly and be shed as quickly as possible is fundamental to water harvesting systems, thermal power generation, distillation towers, etc. However, cutting-edge approaches based on micro/nanoscale textures suffer from intrinsic trade-offs that make it difficult to optimize both growth and transport at once. Here we present a conceptually different design approach based on principles derived from Namib desert beetles, cacti, and pitcher plants that synergistically couples both aspects of condensation and outperforms other synthetic surfaces. Inspired by an unconventional interpretation of the role of the beetle's bump geometry in promoting condensation, we show how to maximize vapor diffusion flux at the apex of convex millimetric bumps by optimizing curvature and shape. Integrating this apex geometry with a widening slope analogous to cactus spines couples rapid drop growth with fast directional transport, by creating a free energy profile that drives the drop down the slope. This coupling is further enhanced by a slippery, pitcher plant-inspired coating that facilitates feedback between coalescence-driven growth and capillary-driven motion. We further observe an unprecedented six-fold higher exponent in growth rate and much faster shedding time compared to other surfaces. We envision that our fundamental understanding and rational design strategy can be applied to a wide range of phase change applications.   

Nucleation pressure threshold in acoustic droplet vaporization

We combine classical nucleation theory with superharmonic focusing to predict necessary pressures to induce nucleation in acoustic droplet vaporization. We show that linear acoustics is a valid approximation to leading order when particle displacements in the sound field are small relative the radius of the droplet. This is done by perturbation analysis of an axisymmetric compressible inviscid flow about a droplet with small surface perturbations relative to the mean radius subjected to an incoming ultrasonic wave. The necessary nucleation pressure threshold inside the droplet is calculated to be $-9.33 \pm 0.30$ MPa for typical experimental parameters by employing results from \textit{classical homogeneous nucleation theory}. As a result we are able to predict if a given incident pressure waveform will induce nucleation.   

Evaporation dynamics of water droplets on inclined surfaces

When a water droplet is gently placed on a flat substrate, particularly which is tilted at an inclined angle, usually there are advancing and receding angles inside the droplet formed by inclination under gravitational force. Evaporation dynamics of an nonspherical inclined droplet at inclinations would deviate from that of a spherical droplet. Here we study on evaporation dynamics rates of inclined droplets by measuring mass changes with time and their lifetimes. We find that the lifetime of an evaporating inclined droplets becomes longer as the gravitational influence becomes stronger. The lifetime depends on the pinning-depinning transitions and the depinning onset times, which are changed by the gravitational influence. This The dependence inclination-induced evaporation behavior would be useful important in understanding evaporation dynamics of inclined droplets.   

Surface temperature measurements of a levitated water drop during laser irradiation

Simulation of high energy laser propagation and scattering in the maritime environment is problematic, due to the high liklihood of turbulence, fog, and rain or sea spray within the beam path. Laser interactions with large water drops (diameters of approximately 1-mm), such as those found in a light rain, have received relatively less attention. In this regime a high energy laser will rapidly heat and vaporize a water drop as it traverses the beam path, but the exact heating / vaporization rate, its dependence on impurities, and ancillary effects on the drop or surroundings are unclear. In this work we present surface temperature measurements of a water drop obtained using a FLIR IR camera. The drop is acoustically levitated, and subject to a continuous wave laser with a wavelength of 1070-nm and a mean irradiance of approximately 500 W/cm$^{\mathrm{2}}$. These measurements show that the steady-state surface temperature of the drop is well below the saturation temperature, yet based on the time history of the drop volume vaporization begins almost immediately upon laser strike. Inferences on the turbulence characteristics within the drop are also made from measurements of the fluctuations in the surface temperature.   

Evaporation of particle-laden droplets on a superhydrophobic surface

We experimentally investigated the evaporation dynamics of water droplets suspended with minute particles of varying concentrations on a superhydrophobic surface. The contact angle, diameter, and height of the droplets decreased during the evaporation process. For pure water, the droplet went through a wetting transition from a partial wetting (Cassie-Baxter), with a large contact angle ($> 140^\circ$), to completely wetting (Wenzel) state, with a small contact angle. Unlike pure water, the nanofluid droplets maintain high contact angles ($> 100^\circ$) during evaporation. We found that the contact line was pinned, and an increase ($10\%$) in the weight fraction of nanoparticles led to a remarkable $40\%$ decrease in the total drying time. The nanofluid droplets left donut-shaped drying patterns. In these final drying structures, a shrinkage of the droplet height and base diameter was observed for nanofluids with lower concentrations. The results show that droplet evaporation rate and deposit pattern depend on the concentration of nanoparticles, implying the crucial influences of water evaporation and particle migration dynamics and time-scales.