Millimeter-sized Drop Research Articles

Space-resolved dynamic light scattering within a millimeter-sized drop: From Brownian diffusion to the swelling of hydrogel beads.

We present a dynamic light scattering setup to probe, with time and space resolution, the microscopic dynamics of soft matter systems confined within millimeter-sized spherical drops. By using an ad hoc optical layout, we tackle the challenges raised by refraction effects due to the unconventional shape of the samples. We first validate the setup by investigating the dynamics of a suspension of Brownian particles. The dynamics measured at different positions in the drop, and hence different scattering angles, are found to be in excellent agreement with those obtained for the same sample in a conventional light scattering setup. We then demonstrate the setup capabilities by investigating a bead made of a polymer hydrogel undergoing swelling. The gel microscopic dynamics exhibit a space dependence that strongly varies with time elapsed since the beginning of swelling. Initially, the dynamics in the periphery of the bead are much faster than in the core, indicative of nonuniform swelling. As the swelling proceeds, the dynamics slow down and become more spatially homogeneous. By comparing the experimental results to numerical and analytical calculations for the dynamics of a homogeneous, purely elastic sphere undergoing swelling, we establish that the mean square displacement of the gel strands deviates from the affine motion inferred from the macroscopic deformation, evolving from fast diffusivelike dynamics at the onset of swelling to slower, yet supradiffusive, rearrangements at later stages.

Capturing the onset of oral processing: Merging of a model food emulsion drop with saliva.

The events occurring before and during the merging of a model liquid food emulsion with saliva have been captured ex vivo using confocal microscopy. In the order of a few seconds, millimeter-sized drops of liquid food and saliva touch and are deformed; the two surfaces eventually collapse, resulting in the merging of the two phases, in a process reminiscent of emulsion droplets coalescing. The model droplets then surge into saliva. Based on this, two distinct stages can be distinguished for the insertion of a liquid food into the oral cavity: A first phase where two intact phases co-exist, and the individual viscosities and saliva-liquid food tribology should be important to texture perception; and a second stage, dominated by the rheological properties of the liquid food-saliva mixture. The importance of the surface properties of saliva and liquid food are highlighted, as they may influence the merging of the two phases.

Distinct coalescence behaviors of hot and cold drops in the presence of a surrounding viscous liquid

Coalescence of a millimeter-sized drop initially touching a pool of the same liquid in the presence of another surrounding viscous liquid is studied in this work, wherein the drop may be hotter or colder than its surroundings. Moreover, the effect of the outer fluid viscosity on the coalescence dynamics and thermal convection is examined. An axisymmetric numerical model is employed to investigate the drop merger dynamics, wherein the drop and pool are modeled as water fluid, and the surroundings are modeled as silicone oils of different viscosities. The coalescence behaviors of hot and cold drops are found to be significantly different, especially at higher temperature differences. An otherwise partial coalescence for an isothermal system turns into a case of total coalescence when the drop is made colder than its surroundings, whereas the behavior in the case of a hot drop does not depart qualitatively from that of a corresponding isothermal system. Thermal convection has been examined in terms of the penetration depth of hot or cold fluid into the pool. Hot drops are found to have a greater penetration depth as compared to cold drops for higher viscosities of the surrounding fluid. The penetration depth is also related to the size of the leading vortex ring and the maximum vorticity magnitude.

Interferometric investigation of the gas-state monolayer of mono-rhamnolipid adsorbing at an oil/water interface

We present a study on biosurfactant uptake at the interface of a millimeter-sized drop of squalene in water, by a recently developed differential interferometric technique. The technique allows detecting capillary waves, with amplitudes of the order of 10−9 m, excited on the surface of the drop by an electric field in the order of 5 V/cm. By studying the resonant surface modes of the drop it is possible to assess the interfacial properties as a function of the surfactant concentration in the water bulk. The technique allows to follow the adsorption process at extremely low surfactant concentration, not accessible by other methods, investigating the gas state in the π-A diagrams of 2D amphiphilic monolayers.

Apparatus to control and visualize the impact of a high-energy laser pulse on a liquid target.

We present an experimental apparatus to control and visualize the response of a liquid target to a laser-induced vaporization. We use a millimeter-sized drop as target and present two liquid-dye solutions that allow a variation of the absorption coefficient of the laser light in the drop by seven orders of magnitude. The excitation source is a Q-switched Nd:YAG laser at its frequency-doubled wavelength emitting nanosecond pulses with energy densities above the local vaporization threshold. The absorption of the laser energy leads to a large-scale liquid motion at time scales that are separated by several orders of magnitude, which we spatiotemporally resolve by a combination of ultra-high-speed and stroboscopic high-resolution imaging in two orthogonal views. Surprisingly, the large-scale liquid motion upon laser impact is completely controlled by the spatial energy distribution obtained by a precise beam-shaping technique. The apparatus demonstrates the potential for accurate and quantitative studies of laser-matter interactions.

Numerical simulation of droplet impact on textured surfaces in a hybrid state

High Weber number drops are found to undergo a Cassie-to-Wenzel transition on hydrophobic textured surfaces. Previous studies with many-body dissipative particle dynamics (MDPD) method were based on an absolute Cassie state, with no splash. Hence, an MDPD method was developed in this paper, which could simulate drop splashes in a hybrid state. Millimeter-sized drops on hydrophobic substrates were simulated with different solid fraction, and results were compared to experimental results using high-speed photography. The numerical spread diameter, contact time, and splash amount are matched with experiments. Results showed that hydrophobic substrates with lower solid fraction possess better water repellency as compared to those with similar apparent contact angle. However, the influence of the microstructure on superhydrophobic surfaces is much less than that on hydrophobic ones, and surfaces with lower solid fraction did not have better water repellency capabilities. It is believed that the MDPD method proposed in this study can effectively predict relationship between surface topography and water repellency of a material.

How microstructures affect air film dynamics prior to drop impact.

When a drop impacts a surface, a dimple can be formed due to the increased air pressure beneath the drop before it wets the surface. We employ a high-speed color interferometry technique to measure the evolution of the air layer profiles under millimeter-sized drops impacting hydrophobic micropatterned surfaces for impact velocities of typically 0.4 m s(-1). We account for the impact phenomena and show the influence of the micropillar spacing and height on the air layer profiles. A decrease in pillar spacing increases the height of the air dimple below the impacting drop. Before complete wetting, when the impacting drop only wets the top of the pillars, the air-droplet interface deforms in between the pillars. For large pillar heights the deformation is larger, but the dimple height is hardly influenced.

On the relationship between ink-jet printing quality of pigment ink and the spreading behavior of ink drops

In this research, we studied the relationship between ink-jet printing quality of lines and the spreading behavior of millimeter-sized ink drops on different types of untreated fabrics. We observed that printing quality depends, to a significant extent, on the spreading behavior of ink, which in turn is influenced by the nature of capillaries present in the yarns. In addition to nature of capillaries, the wettability and bulk structure of the fabric were found to influence drop spreading. Very good correlation was observed between excess line widths of the printed lines and the normalized spreading distance, d/φ* (where d is the drop spreading distance, and φ* is the effective porosity of the fabric). Correlation studies showed that the spreading behavior of millimeter-sized drops can be effectively used to predict the excess line width on transverse threads, which is mainly responsible for poor ink-jet printing quality.

Alumina Splat Investigation: Visualization of Impact and Splat/Substrate Interface for Millimeter-Sized Drops

Coating adhesion-cohesion is strongly linked to the real contact between splats and substrate and between themselves. Unfortunately, the study of a single micrometer-sized splat interface, resulting from flattening and solidification processes and dynamic behaviors, all occurring in a few microseconds, is extremely complex. To overcome problems due to time and dimension scales, many previous works were devoted to the flattening of millimeter-sized drops. However, because of difficulties in producing millimeter-sized ceramic fully melted drops, most works were devoted to metals or alloys. The aim of this work is to present the development of a setup to understand the effect of substrate surface chemistry (oxidation, atom diffusion, etc.) on the flattening of a single millimeter-sized alumina drop. Thus, a new technique to produce such drops with different impact velocities has been developed. It consists in a rotating crucible heated by a transferred arc and a piston controlling substrate velocity and thus drop velocity relative to it. A fast camera (4000 image/s) that combines temperature evolution with a fast pyrometer (4000 Hz), allows following the drop flattening. This system enables the study of interface phenomena (such as desorption of adsorbates and condensates, and liquid drop/substrate wettability) and investigate the effects, at impact, of the kinetic energy or the Weber number of the flattening drop.

Suppression of coalescence by shear and temperature gradients

We describe laboratory experiments on millimeter-sized drops of liquid in air which indicate that both thermocapillary and isothermal shear flows are able to prevent the coalescence of bodies of liquid which would occur readily in the absence of such flows. We have also carried out molecular dynamics computer simulations of nanometer-sized drops, which show the same qualitative behavior in the case of an applied shear. At the other extreme, persistent non-coalescence of larger drops was observed in microgravity conditions in a space shuttle experiment. We give an explanation of the experimental observations based upon lubrication theory and simple continuum hydrodynamics arguments, along with complementary microscopic insight obtained from the molecular simulations.

Laboratory Measurements and Parameterizations of Supercooled Water Skin Temperatures and Bulk Properties of Gyrating Hailstones

Abstract The accretional growth of gyrating hailstones was studied in a pressure- and temperature-controlled icing wind tunnel, starting with oblate ice spheroids, under cloud conditions and at free-fall speeds. Measured parameters were hailstone surface temperature (with infrared radiometric microscope), net collection efficiency, ice fraction, surface roughness, and final shape of the particle. Surface temperatures of water skins covering growing deposits were found to be supercooled up to −2.5°C, depending on experimental conditions. Over the range of variables covered, the transition from solid to spongy ice deposits occurred at surface temperatures below 0°C. This revised Schumann-Ludlam limit is approximated by a linear liquid water content-air temperature relationship. The final shape of the hailstones varied with icing conditions, and different surface roughness characteristics appeared depending on the gyration rates. Other main findings are that spongy deposits grow while millimeter-sized drops ...