Session L30: Drops: Levitation & Particle Laden

Droplet levitating over a moving wall: stability of a lubricating air film

A drop of viscous liquid may levitate when placed over a rapidly-moving inclined solid surface. Here, the droplet's weight is supported by a micrometer-scale air film that develops between the drop and the underlying solid (Sawaguchi et al., JFM 2019). This lubricating air film may develop instabilities, with capillary waves growing and propagating towards the back side of the droplet. We capture this instability experimentally and rationalize its dynamics theoretically.   

Author not Attending

We perform experiments to understand the behavior and speed of droplets larger than the capillary length on inclined surfaces: (i) superhydrophobic and (ii) superheated surface. We consider Newtonian and non-Newtonian fluids to produce droplets with three different viscosities. The kinematics of the droplets when they are gently released down an inclined plane of varying tilt angles are characterized. We find that the non-Newtonian droplets travel at higher velocity compared to the Newtonian at lower tilt angles. Crucially, the velocity of Leidenfrost droplets becomes weakly dependent on viscosity at higher tilt angles, which is in stark contrast with droplets placed on the superhydrophobic surfaces. Using time-resolved imaging and interferometric visualization, we show that at a critical tilt angle, the internal flows of the non-Newtonian drops transition from axisymmetric to asymmetric profiles. This consequently induces sufficiently strong asymmetry in the vapor profile beneath the drop to induce self-propulsion. These findings provide new insights into the kinematics of Leidenfrost droplets, more specifically the rolling and slipping dynamics of droplets, and the mechanism driving the symmetry-breaking phenomenon which gives rise to self-propulsion.

    

Author not Attending

Ambient aerosol particles can be organic and inorganic, with a size range of 10nm to 10 micrometers. These particles impact air quality, clouds, climate, and life on earth by affecting the scattering of sunlight, serving as cloud and ice nucleation nuclei. Classically, these atmospheric aerosol particles are considered a liquid phase; consequently, the viscosity is assumed to be low. Recently, it has been reported that aerosol particles also exist in the form of semi-solid or even amorphous solid-like particles depending on their physical or chemical properties. The information on the particle phase stage is crucial for many atmospheric processes like ice nucleation, water transport, gas-particle interactions, Chemical aging, formation and partitioning of aerosols, and multiphase and heterogeneous reactions.  There are several experimental approaches, such as  Particle rebound, Dimer relaxation, Light scattering, and Shape relaxation, have been developed to probe the viscosity of aerosol particles. However, none of the available techniques is sufficiently versatile to determine aerosol viscosity at atmospherically relevant conditions for various particle sizes, chemical compositions, and sample sizes. Here we present a novel way to measure the viscosity of droplets suspended in an electrodynamic balance under atmospheric conditions. In the present technique, we have used thermodynamically metastable and kinetically stagnant states of Lithium chloride and water droplets at microscopic volumes. The technique involves induced capillary oscillations on the charged droplet using externally applied excitation frequency, and the viscosity is determined by correctly identifying the phase lag between droplet surface oscillations and applied excitation signal. 

    

Heat transfer, vapor diffusion, and Stefan flow around levitating droplets near a heated liquid surface

We consider a slowly condensing droplet levitating near a surface of evaporating liquid and develop a mathematical model to describe diffusion, heat transfer, and fluid flow in the system. The method of separation of variables in bipolar coordinates is used to obtain series expansions for all physical quantities. This framework allows us to determine temperature profiles and condensation rates at the surface of the droplet, as well as calculate the Stokes force for the conditions of levitation. We find that the dependence of the equilibrium concentration on temperature is necessary to accurately model the phase change at the surface of the liquid. The condensation of vapor leads to the temperature in the droplet being, on average, higher than the surrounding air. Due to the temperature gradient in the droplet, condensation is higher at the top of the droplet and lower at the bottom.   

Author not Attending

Sublimating solids when placed on a hot substrate levitate on the cushion of their own vapor. Here we study this so-called Leidenfrost effect for solid carbon dioxide (dry ice) placed on a temperature controlled sapphire substrate. Temporal evolution of the vapor layer thickness between the substrate and dry ice pellet, and the geometric parameters of the latter are captured by an optical system. We observe that the vapor layer thickness for a cylindrical dry ice pellet increases with time as opposed to a volatile puddle of liquid under similar situation. This behavior is also theoretically studied by employing energy and momentum balance under the assumptions of lubrication approximation, uniform vapor layer thickness, and constant pellet diameter. A comparison between the model predictions and experimental data for vapor layer thickness and the pellet height reveal good agreement.   

How rain captures air pollution

Human activities and natural sources pollute the air we breathe, harming our health and the environment, and marring the beauty of our skies.  One of nature’s processes for cleaning the air is rain.  A falling rain droplet sweeps through the air colliding with suspended pollution particles. These particle-droplet collisions have been presumed to be capture events, but the details of what occurs during collision remains unclear. We investigate these collision events and show that rain droplets capture pollution particles internally and on their outer surfaces with multiple collision behaviors, that include: cavity-forming droplet entries, ricochets, and more.  Rain drop diameter and free fall velocity, in addition to pollution particle characteristics determine which capture or escape behavior occurs. Our findings reveals that rain does not capture all particulate matter upon collision nor does it capture all airborne pollutants equally.  Hence, some pollutants may be more difficult to clean out of the air than others and environmental models on rain scavenging efficiencies should take into account both rain and pollution characteristics to more accurately describe pollution fluxes in the environment. 

    

Experimental study of the dynamics of hard and soft colloids during continuous microdroplet formation

Owing to their design flexibility and high interfacial energies, colloidal particles are emerging as alternatives to surfactants as droplet stabilizer (Pickering emulsions)¹. It is well known that surfactants affect drop formation and sizes²; however, the effects of particles and their dynamics inside the droplets and at interfaces during emulsification remain largely unknown. In this work we investigate the dynamics of formation of colloid-stabilised droplets in a flow-focusing microchannel and study the effect of different particle attributes on drop sizes. Silicon oil was used as outer phase, while glycerol/water solutions containing fluorescent polystyrene particles (1 μm or 400 nm), or pNIPAM microgels of equal sizes, were used as droplet phase to study the effects of particle softness, size and concentrations. We used high-speed imaging to capture the dynamics of formation and μLIF to track particle distribution inside the droplets. A reduction of droplet size and formation time was observed with an increase in particle concentration and softness. In both scenarios, the adsorption kinetics are faster thus showing accordance with surfactant-based systems².

 

¹O.S. Deshmukh et al., Adv. Coll. Interf. Sci. 222, (2015).

²M. Kalli et al., J. Coll. Int. Sci. 605 (2022).

    

Colloidal Assembly on Non-Axisymmetric Droplet Interfaces via Electrospray

Microparticles on the interface of a sessile droplet interact via electrostatic and capillary forces governed by particle size, surface charges, and contact line roughness. We created non-spherical droplets using surface energy patterning and delivered microparticles to the interface with electrospray atomization. Using water as the target droplet, we observed the particle assembly over time. We found that the underlying surface energy pattern significantly influenced the colloidal assembly, and drove particles toward the center of the droplet. The particles arranged as a large, single cluster with local hexagonal ordering, but left a clear region free of particles near the contact line. This depletion region is attributed to electrostatic repulsion from the photoresist used to create the surface energy pattern, which retained electric charge from the electrospray. To understand the effect of electrostatic interactions, we explored target droplets with dissimilar dielectric properties. Using patterned substrates and electrospray for particle deposition, we can harness the assembly of colloids at a fluid interface to build repeatable monolayer patterns.   

Particle capture and trapping by large deformable drops in turbulent channel flow

We investigate numerically the capture of neutrally-buoyant, sub-Kolmogorov particles at the interface of large deformable drops in turbulent flow and the subsequent evolution of particle preferential distribution on the drop surface. Direct numerical simulation of turbulence, phase field modeling of the drop interface and Lagrangian particle tracking are used. Excluded-volume interactions, obtained by enforcing particle collisions, are included to prevent unphysical clustering of the trapped particles. Our results show that particles can be captured only if driven towards the drop surface by jet-like turbulent fluid motions. This process is similar to particle deposition at a solid wall and the resulting capture rate can be predicted via a simple mechanistic model that assumes a proportionality between the mass flux of captured particles and the mean concentration of particles that remain afloat in the bulk of the carrier phase. Once captured by the interfacial forces, particles tend to disperse on the surface, and collect into long-term trapping regions where the average surface velocity divergence sampled by the particles is zero. These regions surround the negative-divergence compression regions where particles are driven by the interfacial stresses and determine a much higher surface coverage compared to the case in which excluded-volume interactions are neglected. Such preferential accumulation is a direct consequence of the excluded-volume interactions among neighbouring particles. These regions correlate well with portions of the interface characterized by higher-than-mean curvature, indicating that modifications of the surface tension induced by the presence of tiny particles, and hence of drop deformability, will be stronger in the highly-convex regions of the interface.   

Indoor particulate matter removal using droplet-particle interaction with electric charge effect

Particulate matter (PM) is micro- and nano-sized particles suspended in the air with us. PM has been studied on many adverse effects on human health and environment. Since most people spend 90 % of their time indoors. This is the reason indoor PM removal is important for our healthy life. Various methods for PM removal have been introduced such as filtration, electrostatic precipitation (ESP), and wet scrubbing. However, ESP and wet scrubbing have a limitation to conduct for applying indoor condition because they are focused on specific or harsh environments. In this study, PM removal effects with electric charged droplets generated by electrospraying were experimentally conducted in indoor experimental condition. The electric charge of particles and droplets was measured by Faraday cup to obtain electrostatic effect between particles and droplets. Based on electric charge of particles and droplets, we can successfully predict the deposition constant analytically, and compare the experimental result.