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The fall of a small droplet from a height H = 20 mm onto the surface of a sessile drop leads to a change in its shape: the drop is periodically stretched and then compressed. At H = 500 mm, after the fall of a small droplet, the formation of splashes and fingers is observed. Subsequently, the sessile droplet is compressed with the formation of closely spaced smaller droplets. The PIV method was used to obtain experimental data on the change in velocity in the horizontal section a sessile drop after the fall of a small droplet. The fall of one droplet onto the surface of another drop from a height of H = 20 mm leads to the formation of vortices in the drop and the flow velocity in the drop increases many times from 0.035-0.5 mm/s to 4.1-4.8 mm/s. After 5 s the drop, the flow velocity in the sitting drop approaches the velocity before the fall, and a toroidal vortex is formed in the drop.

Experimental studies for the evaporation of a sessile and hanging droplet of aqueous salt solution were carried out. When spray cooling a hot wall, it is required to simulate both the heat transfer of falling droplets and the heat transfer of sessile droplets on the wall. It is shown that the ratio of the maximum velocity to the average convection velocity is approximately 2.5-3 both for a hanging and a sessile droplet of salt solution.

Experiments on the evaporation of a hanging drop of water in a stream of air were carried out. The velocity for a hanging drop is about 30-40% higher than for a sessile droplet of water. It is shown that the ratio of the maximum convection velocity to the average convection velocity is approximately equal to three, for both a hanging and a sessile droplet of water.

The paper is devoted to the study of the evaporation of water sessile drops resting on a smooth surface consisting of a vanadium dioxide thin film (∼150 nm) covering a sapphire substrate. Vanadium dioxide is considered because of the promising applications it is expected to have in new technologies for heat waste recuperation. Water drops are deposited on the substrate heated at different temperatures from 20 to 80°C, and the time dependence of the droplet shape is registered with the use of shadow imaging. This reveals that the three-phase contact line remains pinned during most of the drop lifetime, while the wetting contact angle as well as the drop volume are continuously changing. With the help of image processing, the vaporization rate of the drop is calculated. It is found that it is almost constant with time, and increases exponentially with the temperature of the substrate, well in agreement with theoretical models developed to describe the evaporation of drops with a pinned contact line.

The coalescence dynamics of an ethanol droplet freely falling on a sessile ethanol droplet is investigated experimentally using a high-speed imaging system. The regime maps showing the partial coalescence and spreading behaviors in the plane of the Weber number (We) and the volume of the sessile droplet (Vp) normalized with the volume of the impacting droplet (Vi) have been presented. The partial coalescence phenomenon is observed when the ratio of the volume of the sessile droplet to that of the impacting droplet (Vp/Vi) is greater than two. For Vp/Vi = 2, the size of the daughter droplet is found to be about 0.1 times as that of the impacting droplet, which increases with the increase in the We and normalized volume of the sessile droplet. In the present study, the negative curvature of the droplet coupled with the presence of the substrate leads to a different coalescence dynamics.

Droplet evaporation under confinement is ubiquitous to multitude of applications such as microfluidics, surface patterning, and ink-jet printing. However, the rich physics governing the universality in the underlying dynamics remains grossly elusive. Here, we bring out hitherto unexplored universal features of the evaporation dynamics of a sessile droplet entrapped in a 3D confined fluidic environment. We show, through extensive set of experiments and theoretical formulations, that the evaporation timescale for such a droplet can be represented by a unique function of the initial conditions. Moreover, using same theoretical considerations, we are able to trace and universally merge the volume evolution history of the droplets along with evaporation lifetimes, irrespective of the extent of confinement. We also showcase the internal flow transitions caused by spatio-temporal variation of evaporation flux due to confinement. These findings may be of profound importance in designing functionalized droplet evaporation devices for emerging engineering and biomedical applications. Published by AIP Publishing.

Evaporation of a sessile droplet often exhibits a mixed evaporation mode, where the contact radius and the contact angle simultaneously vary with time. For sessile water droplets containing polymers with different initial polymer concentrations, we experimentally study their evaporation dynamics by measuring mass and volume changes. We show how diffusion-limited evaporation governs droplet evaporation, regardless of the complexity of evaporation behavior, and how the evaporation rate depends on the polymer concentration. Finally, we suggest a unified expression for a diffusion-limited evaporation rate for a sessile droplet with complexity in evaporation dynamics.

We report on experimental observations/visualization of thermocapillary or Marangoni flows in a pure water drop via infrared thermography. The Marangoni flows were induced by imposing a temperature gradient on the drop by locally heating the substrate directly below the center with a laser. Evidently, a temperature gradient along the liquid-air interface of ca. 2.5 degrees C was required for the Marangoni flows to be initiated as twin vortices and a subsequent gradient of ca. 1.5 degrees C to maintain them. The vortices exhibited an oscillatory behavior where they merged and split in order for the drop to compensate for the non-uniform heating and cooling. The origin of these patterns was identified by comparing the dimensionless Marangoni and Rayleigh numbers, which showed the dominance of the Marangoni convection. This fact was further supported by a second set of experiments where the same flow patterns were observed when the drop was inverted (pendant drop). Published by AIP Publishing.

We report a simple optical technique to measure time-resolved nanoscale surface profile of an evaporating sessile fluid droplet. By analyzing the high contrast Newton-ring like dynamical fringes formed by interfering Fresnel reflections, we demonstrated λ/100 ≈ 5 nm sensitivity in surface height (at 0.01–160 nm/s rate) of an evaporating water drop. The remarkably high sensitivity allowed us to precisely measure its transient surface dynamics during contact-line slips, weak perturbations on the evaporation due to external magnetic field and partial confinement of the drop. Further, we measured evaporation dynamics of a sessile water drop on soft deformable surface to demonstrate wide applicability of this technique.

Cell lysis is a key sample preparation stage in many biomedical studies as DNA extraction and classification require the use of the nucleic acid and proteins released upon decomposition of a cell membrane. We present an effective method of lysing cells suspended in a microliter droplet placed on a super-hydrophobic surface. When a bubble, injected into the sessile droplet, subsequently ruptures, a rapidly moving fluid jet is formed. In this work, cells that are transported within this fluid jet are captured on a separate hydrophilic substrate and are shown to have been lysed.