Session Z30: Drops: Heat Transfer, Evaporation and Buoyancy Effects II

Binary mixture drops evaporation on micro-structured surfaces

In many industrial, biological, and everyday practices, the process of binary drops mixtures evaporation represents tremendous value. In this work, water-ethanol binary mixture drops are studied on smooth and on structured surfaces. On structured surfaces, three different evaporative behaviours have been observed; pinning or constant contact radius, stick-slip and mixed mode, which are consistent with literature. In addition, there are two further unexpected evaporation modes, which we report for the first time. We describe this as "increasing contact angle mixed-stick-slip" (ICAMSS) mode due to the preferential evaporation of ethanol and presence of structures and "decreasing contact angle mixed-stick-slip" (DCAMSS) mode due to the presence of structures. The duration of each evaporation mode is then discussed while evaporation regime maps are proposed. Regime maps show that high ethanol concentration drops on short spacing structures tend to evaporate mostly in a pinned mode while the ICAMSS mode ensues on larger spacing structures. High water concentration drops on large spacing structures tend to evaporate mostly in a stick-slip and DCAMSS modes. In conclusion, tailoring evaporation regimes can help in choosing the appropriate liquid and surface structure for particular applications, which may prove beneficial to many patterning and biomedical applications.   

Gravity-driven evaporative flux profiles of a sessile droplet

Droplet evaporation is a universal and important problem for various industrial fields such as inkjet printing, agriculture, spray cooling, and DNA microarray. It is well-known that the evaporative flux is non-uniform along the droplet surface and the flux is divergent at the contact line, which induces a coffee-ring flow. However, it is still difficult to directly measure the evaporative flux of the drying droplet. Hence, most of researchers typically investigated how the internal flow patterns look like when the droplet evaporated in various situations. Due to this reason, vapor distribution around an evaporating droplet has rarely been studied. In this work, we directly visualized the evaporated vapors of highly volatile liquid using a laser interferometry. We found that the evaporative flux and speed are dramatically suppressed or enhanced depending on the droplet position, e.g., on-plate and upside-down. We confirmed that the vapor distribution plays an important role to determine the total evaporation time. 

    

Frost Formation on Macro-Textured Surfaces with a Temperature Gradient

Frost formed on surfaces with temperature variations causes structural and energy efficiency problems in different sectors such as transportation, heat exchangers, and wind power industries. To understand and control the frost formation on three-dimensional surface structures with a temperature gradient, we tested surfaces with macro-textures such as arrays of cylindrical and conical pillar structures. These textured surfaces enable a trade-off condition between two gradients that induce (or inhibit) frost formation: thermal and chemical. When cooled from either side, a temperature and non-uniform water vapor flux distribution across the surface will result. The thermal and chemical gradients determine where the frost will first form, and how far the frost front will advance. Experimentally and numerically, we analyzed the location of the local critical point on the side wall of the macro-textures separating the frost covered and uncovered sections. The numerical calculations were then validated by experimental studies employing the structures. This research will further provide an essential set of surface design parameters for anti-frost surface design under forced convection conditions.

    

Understanding the Effects of Heat Transfer on Snow Removal from Photovoltaic Modules

Snow accumulation on photovoltaic (PV) modules causes damage on them and significant financial losses by preventing their power generation. Here, snow removal mechanisms are studied under different heat transfer modes. A snowphobic coating (details of which is not provided) is developed and applied on small glass/aluminium surfaces mimicking PV modules to facilitate snow removal. Artificial snow with controlled liquid water content (LWC) is formed using a snow gun that mixes compressed air and water from a faucet. The LWC of the created snow is monitored using a surface-mountable sensor. The effects of free convection and shortwave radiation on snow removal are studied. Under free convection, the aluminium/glass surface with the coating allowed snow removal 25 hours earlier than the control bare sample. Under shortwave radiation, the coating resulted in 3 hours faster snow removal. Under a combination of convection and radiation, the coating allowed snow removal 3.5 hours earlier than the bare sample. Under outdoor natural snow conditions, the coating significantly improved snow removal by clearing snow off the PV module 10 days before the bare module. This study allows for developing a combination of active and passive strategies to facilitate snow removal from PV modules to maximize their power generation throughout the winter season.   

Droplet evaporation on inclined patterned substrates

The evaporation of droplets on inclined substrates is important for a broad range of applications, including ink-jet printing, surface cooling, and micro-structure assembly. Despite its apparent simplicity, the precise configuration of an evaporating droplet on a solid surface has proven notoriously difficult to predict and control. Here, we study the effect of smooth patterns on a droplet evaporating on an inclined plane. The evaporation is assumed to be quasi-static so that the dynamics of the droplet can be quantified by the equilibrium properties of the system. We first examine the limiting case of small gravity (small Bond number) and perform asymptotic analysis to obtain approximate solutions. For higher Bond numbers we derive exact solutions written in terms of elliptic integrals that can be solved numerically. By studying the stability of these solutions, and how it depends with the droplet size, we perform a bifurcation analysis that shows the emergence of a hierarchy of bifurcations that strongly depends on the particular underlying chemical pattern. We show in turn that gravity affects the topology of the bifurcation diagrams, reducing the stability regions of the phase space. To understand the droplet dynamics upon evaporation on inclined surfaces, we perform numerical simulations of the Cahn-Hilliard and Navier-Stokes system of equations. We focus on the quasi-static regime, where droplet evaporation is dominated by diffusion into the gas phase, and the density contrast between fluids is taken into account via a Boussinesq approximation. We observe very good agreement between the numerical results and the behaviour predicted by the quasi-static analysis. Our results show that the interplay between a phase change and surface wettability can be exploited to control the motion of droplets on inclined patterned solid surfaces.   

Author not Attending

The evaporation of sessile droplets occurs in numerous physical contexts, with applications in nature, industry and biology, including nano-fabrication, chemical decontamination, and ink-jet printing. As a consequence of the wide variety of everyday applications, the evolution of and deposition from an evaporating sessile droplet has been subject to extensive investigation in recent years, with particular interest in different evaporation modes, the prediction of lifetimes, and the ring-like deposit (the "coffee-ring") which often forms at the contact line of a pinned evaporating droplet. In this talk we report on some of the recent developments in the study of evaporating droplets at the University of Strathclyde. We first formulate and analyse a mathematical model for the evolution and lifetime of an evaporating droplet in a well of rather general shape and validate the model by comparing the theoretical predictions with experimental results for the special case of cylindrical wells. (This part of the work was done in collaboration with Durham University, and further details of it can be found in the recent paper by D'Ambrosio et al. J. Fluid Mech. 927 A43 (2021).) We then investigate the effect of the spatial variation in the local evaporative flux on the deposition from an evaporating droplet, determining the flow velocity, concentration of solute within the droplet, and the evolution of the deposit for a one-parameter family of evaporative fluxes. Finally we discuss more recent work exploring the effect of gravity on the shape and evolution of evaporating sessile and pendant droplets, and the effect of particle adsorption on the deposition from an evaporating sessile droplet.   

Quantifying the interacting mechanisms in shape evolution of sessile volatile droplets

Non-uniform distribution of interfacial mass flux across an evaporating/condensing droplet will subsequently induce a temperature gradient across the liquid-air interface, and result in thermal Marangoni stress that reforms the flow field inside the droplet. Recent study (Shiri, et al. Phys. Rev. Lett. 2021, Yang, et al. Langmuir 2022) confirmed the impact of thermal Marangoni flow on the shape of single component volatile droplets on completely wetting substrates. Nevertheless, a comprehensive evaluation of the interacting mechanisms is still lacking due to the intrinsic limitation of experimental techniques to decompose the different influence factors. To this end, we formulate a mathematical model to simulate the evaporation/condensation of a single component droplet on completely wetting substrates accounting for the thermal transport across the solid substrate. With this model, we elucidate the interacting physical mechanisms that govern the droplet kinetics during phase change, which include the capillary effect, the thermal Marangoni effect, the interface motion due to evaporation/condensation, as well as the removal of energy barriers by precursor film. Taking the thermophysical properties of water as the base parameters, we elucidate that the thermal Marangoni effect plays a non-negligible role in the evolution of droplet geometry during phase change. Additionally, the liquid volatility along with the substrate conductivity and thickness influence the magnitude of the thermal Marangoni stress. A phase diagram is subsequently summarized in terms of the Knudsen number, which compares the interfacial thermal effect to the liquid phase thermal transport, and the Biot number, which compares the the heat transfer through the liquid phase and through the solid phase. The conclusions clearly elucidate the prevalence and interaction of the Marangoni and capillary effects on droplet dynamics with interfacial phase change.   

Author not Attending

Liquid-solid interactions are ubiquitous in numerous naturally occurring phenomena and industrial processes. Over the past few years, a particular interest has been given to understanding the characteristics and dynamics of liquid drop impact on heated substrates maintained above the Leidenfrost temperature, specifically in applications that require favourable conditions for efficient cooling. Upon impact, the drop exhibits a vigorous phase-change behaviour coupled with the inception of an intervening vapour layer leading to a drastic reduction in the heat transfer between the liquid and surface. Interestingly, the strong interfacial oscillations at the liquid-vapour interface give rise to rich dynamics and pattern formation, excited mainly by the incessant development and discharge of vapour pockets beneath the drop. Additionally, bubble entrapment and jetting are other unique manifestations of Leidenfrost drops attributed to the convergence of capillary waves at the liquid-vapour interface. This study employs an in-house solver for Direct Numerical Simulations (DNS) based on the Level Contour Reconstruction Method (LCRM) to accurately capture the highly deforming liquid-vapour interface. We determine the onset of the interfacial oscillations, demonstrate the singularities leading to vapour bubble entrapment, and validate the occurrence of upward and/or downward jetting.   

Influence of Microgravity and Marangoni Flow on the Vapor Distribution Surrounding an Evaporating Sessile Droplet

In microgravity, thermal Marangoni flow in an evaporating sessile droplet of a pure volatile liquid is expected to be dominant enough to affect advection in the gas and hence to alter the vapor distribution around the droplet and the evaporation rates. In order to test this hypothesis, the vapor cloud surrounding a hydrofluoroether (HFE-7100) sessile droplet has been visualized using a Mach-Zehnder interferometer. The microgravity conditions were produced in a series of parabolic flight manoeuvres organized by ESA. During the tests, the phase-wrapped images obtained from vapor interferometry show that the vapor cloud is not only influenced by the Marangoni flow but also by the inherent g-jitters arising from the flight itself. However, the impact of g-jitters is observed to be significant only during the later stages of the microgravity phase. The vapor cloud, initially hemispherical, rises and forms a plume-like structure. Although there is interference from the g-jitters, this plume is mostly a consequence of a fully developed Marangoni convection inside the droplet. The evaporation rates were measured by tracking the droplet volume evolution. On the contrary, the experiments conducted on ground showed a vapor cloud flattened by gravity with different evaporation rates.   

Evaporation of Acoustically Levitated Ouzo Droplets

Ouzo droplet is a multi-component droplet formed by liquids with varying thermophysical properties, volatilities, and miscibility. The evaporation process of ouzo droplets is a spontaneous emulsification process and is a complex multi-physics problem involving fascinating mass and heat transfer phenomena, as well as phase-changing behavior, recently has been drawn significant attention. In this study, evaporation of multi-component droplets composed of ethanol, water, and anise oil is investigated experimentally using acoustic levitation and infrared thermography techniques. The normalized surface area and temperature data are investigated simultaneously to classify the evaporation into five distinct stages. Scaling laws reveal the self-similar evaporation behavior for different size droplets. A numerical model has been deployed to provide additional insights revealing the physics behind the complex evaporation behavior.