We investigate the evaporation dynamics of a sessile droplet of ethanol-water binary mixtures of different compositions laden with alumina nanoparticles and compare with the no-loading condition at different substrate temperatures. Shadowgraphy and infrared imaging methods are used, and the experimental images are postprocessed using a machine learning technique. We found that the loading and no-loading cases display distinct wetting and contact angle dynamics. Although the wetting diameter of a droplet decreases monotonically in the absence of loading, the droplet with 0.6 wt % nanoparticle loading remains pinned for the majority of its lifetime. The temporal variation of the normalized droplet volume in the no-loading case has two distinct slopes, with ethanol and water phases dominating the early and late stages of evaporation, respectively. The normalized droplet volume with 0.6 wt % loading displays a nearly linear behavior because of the increase in the heat transfer rate. Our results from infrared imaging reveal that a nanofluid droplet displays far richer thermal patterns than a droplet without nanoparticle loading. In nanoparticle-laden droplets, the pinning effect, as well as the resulting thermo-capillary and thermo-solutal convection, causes more intense internal mixing and a faster evaporation rate. Finally, a theoretical model is also developed that satisfactorily predicts the evaporation dynamics of binary nanofluid droplets.

Evaporation Dynamics of a Sessile Droplet of Binary Mixture Laden with Nanoparticles.

The coalescence of viscous drops on a substrate is studied experimentally and theoretically. We consider cases where the drops can have different contact angles, leading to a very asymmetric coalescence process. Side view experiments reveal that the "bridge" connecting the drops evolves with self-similar dynamics, providing a new perspective on the coalescence of sessile drops. We show that the universal shape of the bridge is accurately described by similarity solutions of the one-dimensional lubrication equation. Our theory predicts that, once the drops are connected on a microscopic scale, the bridge grows linearly in time with a strong dependence on the contact angles. Without any adjustable parameters, we find quantitative agreement with all experiments.

Symmetric and Asymmetric Coalescence of Drops on a Substrate

We investigate the dynamics of two immiscible fluids in a cross-junction via three-dimensional numerical simulations using the volume of fluid approach to track the dispersed phase's evolution. Different regimes, namely the dripping, squeezing, and jetting dynamics, have been observed for different dimensionless parameters, and we unveil a transition in regimes due to the concomitant interplay of capillarity, viscosity, and wettability. Our results reveal that hydrophobic channel surfaces favor a transition from squeezing to dripping behavior at a lower value of the capillary number. Moreover, higher viscosity ratios advance the process of squeezing, necking, and breakage on hydrophobic surfaces. A wettability–capillarity regime map is also presented that will have significant implications regarding the choice of substrate wettability, fluid properties, and flow rate in droplet dispensing devices.

Tuning of regimes during two-phase flow through a cross-junction

Coalescence of argon droplets with a radius of 25, 50, and 100 nm is studied with computational methods. Molecular dynamics (MD) simulations are carried out to generate reference data. Moreover, a phase-field model resting on a Helmholtz energy equation of state is devised and evaluated by computational fluid dynamics (CFD) simulations. Exactly the same scenarios in terms of geometry, fluid, and state are considered with these approaches. The MD and CFD simulation results show an excellent agreement over the entire coalescence process, including the decay of the inertia-induced oscillation of the merged droplet. Theoretical knowledge about the asymptotic behavior of coalescence process regimes is confirmed. All considered scenarios cross from the inertially limited viscous regime over to the inertial regime because of the low shear viscosity of argon. The particularly rapid dynamics during the initial stages of the coalescence process in the thermal regime is also captured by the phase-field model, where a closer look at the liquid density reveals that metastable states associated with negative pressure are attained in the emerging liquid bridge between the coalescing droplets. This demonstrates that this model is even capable of adequately handling the onset of coalescence. To speed up CFD simulations, the phase-field model is transferred to coarser grids through an interface widening approach that retains the thermodynamic properties including the surface tension.

Droplet coalescence by molecular dynamics and phase-field modeling

An explicit conservative Saul’yev scheme for the Cahn–Hilliard equation

We investigate the dynamics of a self-rewetting drop placed on a substrate with a constant temperature gradient via three-dimensional numerical simulations using a conservative level-set approach to track the interface of the drop The surface tension of a so-called self-rewetting fluid exhibits a parabolic dependence on temperature with a well-defined minimum Two distinct drop behaviours, namely deformation and elongation, are observed when it is placed at the location of the minimum surface tension The drop spreads slightly and reaches a pseudo-steady state in the deformation regime, while it continuously spreads until breakup in the elongation regime Theoretical models based on the forces exerted on the drop have been developed to predict the critical condition at which the drop undergoes the transition between the two regimes, and the predictions are in good agreement with the numerical results We also investigate the effect of the initial position of the drop with respect to the location of the minimum surface tension on the spreading and migration dynamics It is found that, at early times, the migration of the drop obeys an exponential function with time, but it diverges at the later stage due to an increase in the drop deformation

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/AB7F6EB9AD4BBB353DDC37936539961B/S0022112021001300a.pdf/div-class-title-motion-of-self-rewetting-drop-on-a-substrate-with-a-constant-temperature-gradient-div.pdf

Jun-Yuan Chen

Papers

Motion of self-rewetting drop on a substrate with a constant temperature gradient

Early stage of delayed coalescence of soluble paired droplets: A numerical study