Session M13: Drop Evaporation

The effect of electrostatic forces on the evaporation of liquid droplets

The current study seeks to understand the effects of electrostatic forces on liquid droplets deposited on uniformly heated substrates. The electrostatic forces arise due to interactions between charges in the substrate and ions in the liquid in the absence of an electric field. The mathematical model uses an axisymmetric thin film approximation considering the effects of surface tension, evaporation into pure vapor, thermocapillarity, gravity, London-van der Waals disjoining pressure, Marangoni forces, as well as electrostatic forces. The electrostatic forces are added by incorporating the nonlinear Poisson-Boltzmann equation and measured by the electric Weber number. We find that increasing the electric Weber number leads to a decrease in evaporation time as the interface shape and therefore evaporative flux changes. We would like to acknowledge the generous contribution of NASA in funding this research.    

Effect of temperature gradient on the substrate on the coffee-ring dimensions

We experimentally investigate the coffee-ring deposits obtained after evaporation of a 1.1 µl water droplet containing polystyrene colloidal particles (0.3 µm diameter) with an imposed temperature gradient on the surface of the glass substrate. The temperature gradient was achieved by using two thermoelectric coolers underneath the substrate and was confirmed by the infrared thermal camera measurements. High-speed visualization was employed to record the time-varying droplet evaporation. The three substrate temperature gradients obtained were 3.5 °C/mm, 2.4 °C/mm, and 1.3 °C/mm. The deposits are qualitatively visualized under an optical microscope, and scanning electron microscope and are compared with the deposits obtained on constant temperature substrate. The measurements showed that there is a substantial difference in the ring width on the hot side ring perimeter than the cold side ring perimeter. Due to the imposed temperature gradient, the hot side droplet is at lower surface tension than the cold side and the Marangoni convection loop changes inside the droplet into a single loop unlike the two loops on a constant temperature substrate. The fluid and the particles flow from the hot side to the cold side along the droplet surface.   

Measuring Vapor Concentration and Diffusive Flux Distributions of an Evaporating Drop

Measurements of the distribution of the vapor concentration surrounding an evaporating sessile drop are important for revealing the nature of the vapor phase transport mechanisms, which for many conditions controls the evaporation rate and can affect the liquid phase transport. From the measured concentration distribution, the gradient can be computed to yield the diffusive flux distribution throughout the vapor field. This presentation will explain the experiments to measure the vapor distribution, the computational simulation of the measurements which is used to analyze the effects of the propagation of uncertainties caused by measurement noise, and the digital filtering technique to reduce the uncertainties. The measured vapor concentration and gradient distributions surrounding an evaporating methanol drop are presented and those distributions indicate that vapor phase convection is a significant transport mechanism.
  

Snap evaporation of droplets on smooth topographies

The evaporation of droplets on solid surfaces is important for a broad range of applications, including ink-jet printing and surface cooling. Despite its apparent simplicity, the precise configuration of an evaporating droplet on a solid surface has proven notoriously difficult to predict and control. This is because droplet evaporation typically proceeds as a `stick-slip' sequence (a combination of pinning and de-pinning events) caused by microscopic structure of the solid surface. Here we show how smooth, pinning-free, solid surfaces of non-planar topography give rise to a different process called snap evaporation. During snap evaporation the morphology of an evaporating droplet follows a reproducible sequence of steps, where the liquid-gas interface is quasi-statically reduced by mass diffusion until it undergoes an out-of-equilibrium snap. Experimentally, we demonstrate this process using a water droplet evaporating on a wavy ultra-smooth lubricant-infused surface. Mathematically, we use full hydrodynamics lattice-Boltzmann simulations, and a model based on bifurcation theory that reveals the points where snap events are triggered, which obey a strict hierarchy dictated by the underlying surface topography.   

Simulation of laser-induced equilibrium drop evaporation

In a maritime environment high energy lasers frequently interact with liquid water in the form of fog, rain, or sea spray. From these interactions the laser propagation is disturbed, but at high powers the medium is also disturbed due to laser heating and liquid vaporization. Using COMSOL Multiphysics, high energy laser interaction with a single large water drop is explored in a regime where the heat rate from evaporation is in equilibrium with the energy absorption rate from the laser. Peak liquid temperatures from the drop interior are assessed and are compared with saturation temperatures to predict the onset of nucleate boiling. Simulated surface temperatures and evaporation rates are validated against experimental data from laser interactions with single acoustically levitated water drops.    

The influence of wall superheat and drop impact parameters on heat transfer during drop impact on a hot wall

In the last decades numerous experimental and numerical studies have been carried out addressing the heat transfer during drop impact on a hot wall. This phenomena occurs in variety of applications such as fuel injection into combustion engines and spray cooling. To obtain a detailed understanding of the heat and mass transfer mechanisms during drop impact, an experimental study is essential. Embedded on this background, the main objective of this study is to experimentally investigate the influence of wall superheat as well as drop impact size and velocity on heat transfer during drop impact on a hot wall. The experimental setup consists of a temperature-controlled test cell, which is maintained under the saturated condition of working fluid FC-72. A drop is generated inside the cell thanks to a syringe pump. This experiment is making use of high spatial and temporal resolution thermography and shadowgraphy techniques to measure the temperature field as well as to visualize the drop hydrodynamics. Various drop sizes and impact velocities can be achieved through varying the needle size and height, respectively. Distinctive wall superheat is obtained by passing various current through the heater. The experimental results are validated with an existing model.