Session Q12: Drops: Heat Transfer, Evaporation and Buoyancy Effects I

Evaporation of alcohol droplets in humid environments

It is common to spray surfaces with alcohol to remove any potential source of infection. Once on the surface, the alcohol droplets quickly evaporate and leave behind a dry and cleaner surface. Here, we use experimental observations and theoretical modeling to show that this simple daily observation hides a wealth of fascinating fluid dynamics. We show that while the alcohol droplet evaporates, water condenses into the drop and later evaporates again. At low ambient relative humidities, the condensation of water does not have a considerable effect on the dynamics of the evaporating alcohol drop. However, beyond a critical ambient relative humidity, condensation of water creates strong solutal Marangoni flows that significantly deform the drop, making it pancake-like in shape. We show that at these high relative humidities the alcohol content of the drop quickly decays, leaving behind a thin film of water on the surface that evaporates much more slowly. Our observations suggest that when evaluating the effectiveness of alcoholic sprays for disinfecting surfaces it is necessary to account for the ambient relative humidity.   

Droplet evaporation on inclined substrates

Drying of droplets on inclined substrates is relevant to applications such as spray coating, ink-jet printing, and crime-scene reconstruction. Recent experiments have shown that steeper substrate inclination can significantly slow down droplet evaporation due to faster droplet depinning. Motivated by these experiments, we develop a lubrication-theory-based model to study the effect of substrate inclination on the evaporation of two-dimensional pure-solvent and particle-laden droplets on smooth and rough inclined substrates. A system of partial differential equations describing the time evolution of the droplet thickness and the colloidal particle concentration is derived and then solved with a finite-difference method. The contact-line motion is described with a disjoining-pressure/precursor-film approach and the well-known one-sided model is used to describe solvent evaporation. Our results indicate that on smooth substrates steeper inclination speeds up evaporation due to larger deformation of the droplet interface, which leads to a smaller droplet thickness. On rough substrates, the effect of substrate inclination on evaporation depends on the Bond number (Bo). At lower Bo, steeper substrate inclination slows down evaporation due to faster droplet depinning, which leads to a larger droplet thickness. At higher Bo, the droplet does not pin due to stronger gravitational forces, and steeper substrate inclination speeds up evaporation, similar to smooth substrates. When colloidal particles are present, the resulting final particle deposition patterns are strongly dependent on the initial conditions (Bo, inclination angle, initial particle concentration, wettability) rather than being a function of only substrate inclination. Our model predictions qualitatively agree with previous experimental work and disentangle the effects of evaporation, substrate inclination, and surface roughness.   

The dynamics of sessile drop during evaporation on a heated substrate under microgravity

We have explored the dynamics of a sessile drop on a heated substrate under microgravity. Microgravity helps us to quantify and distinguish the influence of natural convection on the evaporation rate. By leverage of the microgravity condition, a numerical model have been developed to better understand the drop dynamics that take place during the evaporation process. Quantitative validation of our model has been performed by comparing it to experimental results. We suggest a correlation for the evaporation rate of an Ethanol sessile drop that accounts for relevant physical parameters of the problem. Simultaneously, we provide an analysis of the flow motion with 3D resolved computations for the whole lifetime of evaporating sessile drop. Thanks to this analysis, we provide an insight into the Marangoni effect in the dynamics of the evaporation process and the occurrence of secondary Marangoni instability. We capture the fine influences of the secondary instabilities appearance on the evaporation rate. Concurrently, we define the instability threshold for the transition from primary to secondary Marangoni instability via thermal Marangoni number and geometrical aspect ratio.   

Sessile volatile drop evaporation under microgravity

The evaporation of sessile drops of various volatile and non-volatile liquids, and their internal flow patterns with or without instabilities have been the subject of many investigations. The current experiment is a preparatory one for a space experiment planned to be installed in the European Drawer Rack 2 (EDR-2) of the International Space Station (ISS), to investigate drop evaporation in weightlessness. In this work, we concentrate on preliminary experimental results for the evaporation of hydrofluoroether (HFE-7100) sessile drops in a sounding rocket that has been performed in the frame of the MASER-14 Sounding Rocket Campaign, providing the science team with the opportunity to test the module and perform the experiment in microgravity for six consecutive minutes. The focus is on the evaporation rate, experimentally observed thermo-capillary instabilities, and the depinning process. The experimental results provide evidence for the relationship between thermo-capillary instabilities and the measured critical height of the sessile drop interface. There is also evidence of the effects of microgravity and Earth conditions on the sessile drop evaporation rate, and the shape of the sessile drop interface and its influence on the de-pinning process.   

Measurement of acetone-vapor concentration field using Background oriented schlieren (BOS)

Droplet evaporation is a simple natural phenomenon, however, associated with complex heat and mass transfer processes and seen in a wide range of applications, e.g. spray cooling, medical treatment, inkjet printing, etc. Therefore, it is important to understand the inherent coupling between the temperature and the vapor concentration to control the evaporation process and develop more accurate heat transfer models to predict evaporation rates. The present work demonstrates the application of background-oriented schlieren (BOS) to measure the transient concentration gradient around the evaporating acetone droplet during its impact on a horizontally placed aluminium surface at room temperature and atmospheric conditions. From the concentration maps retrieved, the droplet evaporation is identified to be primarily driven by the concentration difference. The experiments also revealed the relative dominance of the contact line evaporation process after the initial transients seen in the form of a significant increase in the concentration of acetone vapor in the vicinity of the contact line.   

Selective evaporation at the nozzle exit leads to changes in the resonance behavior and an evaporative equilibrium in the piezoacoustic ink channel

In piezo acoustic drop-on-demand inkjet printing a single droplet is produced for each piezo driving pulse. The inks used in most applications are multicomponent, leading to selective evaporation at the nozzle exit during idling periods of the printhead. Here, we study the change in the water-glycerol composition in the nozzle due to the evaporation at the nozzle exit by measuring the ring-down signal of the acoustics in the ink channel. We find excellent agreement between the experiments and the analytical model, showing that the changes in the acoustic resonance frequency of the ink channel are a result of changes in both density and viscosity of the fluid within the nozzle of the printhead. Using this measurement method we observe that the evaporation process reaches an equilibrium situation with respect to the waiting time, where the evaporating water at the nozzle exit is diffusively replenished by water from the ink channel reservoir, as we demonstrate through direct numerical simulations.   

Transient growth stability analysis of evaporating sessile drops comprising binary mixtures

The evaporation and spreading dynamics of a binary mixture sessile drop are complex due to the interplay of thermal and solutal Marangoni stresses alongside the hydrodynamic transport, evaporation, mass diffusion and capillary stress of the drop. Under the assumption of an ideal binary mixture and linear surface tension dependence on temperature and concentration, we recently investigated the quasi-steady linear stability of volatile bicomponent sessile drops with high wettability comprising ethanol-water deposited on heated substrates.  The stress singularity at the contact line is avoided by including a precursor film. Our analysis shows several competing modes for bicomponent drops – indicating that the quasi-steady analysis may not be suitable. To understand the roles of these modes better, we have developed a transient growth analysis. This linear stability analysis applies small disturbances to the binary system, giving perturbed stability equations that evolve with time. These are solved alongside the base state to find the linear stability growth characteristics.   

The Leidenfrost effect as a directed percolation phase transition

Volatile drops deposited on a hot solid can levitate on a cushion of their own vapor, without contacting the surface. We propose to understand the onset of this so-called Leidenfrost effect through an analogy to non-equilibrium systems exhibiting a directed percolation phase transition. When performing impacts on superheated solids, we observe a regime of spatiotemporal intermittency in which localized wet patches coexist with dry regions on the substrate. We report a critical surface temperature, which marks the upper bound of a large range of temperatures in which levitation and contact coexist. In this range, with decreasing temperature, the equilibrium wet fraction increases continuously from zero to one. Also, the statistical properties of the spatio-temporally intermittent regime are in agreement with that of the directed percolation universality class. This analogy allows us to redefine the Leidenfrost temperature and shed light on the physical mechanisms governing the transition to the Leidenfrost state.   

Modeling the Influence of Inertial Clustering on the Evolution and Evaporation of Droplet Clouds

It is known that in a turbulent environment and under certain conditions, inertial particles that were initially randomly distributed tend to cluster. This phenomenon is commonly neglected in stochastic droplet simulations, in spite of being relevant in many applications where e.g. clustering significantly reduces the evaporation or condensation rate of droplets.

In this work we study statistical properties of the gas flow seen by dispersed evaporating droplets in homogeneous, isotropic, stationary turbulence. An accurate description of properties seen by particles is essential in any numerical method relying on the point-particle assumption, whether it is in the context of Reynolds Averaged Navier-Stokes or Large Eddy Simulations.

Guided by data from Direct Numerical Simulations (Weiss et. al, Physics of Fluids 30, 2018) at different flow conditions, we present a hierarchy of models ranging from simple evaporation models based on mean seen quantities to detailed Lagrangian models that account for droplet clustering. The latter incorporate in addition to droplet dynamics the joint statistics of droplet diameter and seen temperature/saturation leading to predictions of superior accuracy.   

Dynamics of Evaporating Aerosols in the Vicinity of Vortical Flows

The dispersion and evaporation of droplets are fundamental processes in aerosol science and are

essential in many atmospheric, environmental, and medical flows, including the spread of

pathogens. Almost any relative air movement about an object is expected to create vortical flow

structures in its wake, depending on its geometry and flow conditions. Therefore, a new

mathematical analysis was carried out, studying the dynamics and dispersion of aerosols under the

influence of approaching vortical flow structures. Our theoretical analysis reveals non-intuitive

interactions between the vortical structure, particle relaxation time, gravity, and mass transfer rates.

The existence of optimal conditions for maximum settling distances is suggested, where the aerosol

entrainment reaches up to an order of magnitude larger than the vortex core length scales.

Phenomena that are not directly linked to vortical flows were isolated and thus the fundamental

influence of the carrier flow over the aerosol entrainment is investigated.