Direct visualization of particle attachment to a pendant drop.

An experimental investigation is carried out into the attachment of a single particle to a liquid drop. High-speed videography is used to directly visualize the so-called 'snap-in' effect which occurs rapidly over sub-millisecond timescales. Using high-magnification, the evolution of the contact line around the particle is tracked and dynamic features such as the contact angle, wetted radius and force are extracted from these images to help build a fundamental understanding of the process. By examining the wetted length in terms of an arc angle, ϕ, it is shown that the early wetting stage is an inertial-dominated process and best described by a power law relation, i.e. ϕ ∼ (t/τ)α, where τ is an inertial timescale. For the subsequent lift-off stage, the initial particle displacement is matched with that predicted using a simple balance between particle weight and capillary force with reasonable agreement. The lift-off force is shown to be on the order of 1-100 µN, whilst the force of impacting droplets is known to be on the order of 10-1000 mN. This explains the ease in which liquid marbles are formed during impact experiments.

Millimetric granular craters from pulsed laser ablation.

This Rapid Communication reports on an experimental study of granular craters formed by a mechanism, namely, optical energy, via a pulsed laser focused onto the surface of a granular bed. This represents an insight into granular cratering for two reasons; first, there is no physical contact between the initiation mechanism and the granular media (as typical for impact or explosion craters). Second, the resulting craters are millimetric in scale, which facilitates a test of energy scalings down to a previously unobserved lengthscale. Indeed, we observe a range of energy scalings conforming to D_{c}∼E^{ß} with ß≈0.31-0.43 depending on the characteristics of the granular media.

Impact dynamics of particle-coated droplets.

We present findings from an experimental study of the impact of liquid marbles onto solid surfaces. Using dual-view high-speed imaging, we reveal details of the impact dynamics previously not reported. During the spreading stage it is observed that particles at the surface flow rapidly to the periphery of the drop, i.e., the lamella. We characterize the spreading with the maximum spread diameter, comparing to impacts of pure liquid droplets. The principal result is a power-law scaling for the normalized maximum spread in terms of the impact Weber number, D_{max}/D_{0}∼We^{α}, with α≈1/3. However, the best description of the spreading is obtained by considering a total energy balance, in a similar fashion to Pasandideh-Fard et al. [Phys. Fluids 8, 650 (1996)]PHFLE61070-663110.1063/1.868850. By using hydrophilic target surfaces, the marble integrity is lost even for moderate impact speeds as the particles at the surface separate and allow liquid-solid contact to occur. Remarkably, however, we observe no significant difference in the maximum spread between hydrophobic and hydrophilic targets, which is rationalized by the presence of the particles. Finally, for the finest particles used, we observe the formation of nonspherical arrested shapes after retraction and rebound from hydrophobic surfaces, which is quantified by a circularity measurement of the side profiles.

Craters produced by explosions in a granular medium.

We report on an experimental investigation of craters generated by explosions at the surface of a model granular bed. Following the initial blast, a pressure wave propagates through the bed, producing high-speed ejecta of grains and ultimately a crater. We analyzed the crater morphology in the context of large-scale explosions and other cratering processes. The process was analyzed in the context of large-scale explosions, and the crater morphology was compared with those resulting from other cratering processes in the same energy range. From this comparison, we deduce that craters formed through different mechanisms can exhibit fine surface features depending on their origin, at least at the laboratory scale. Moreover, unlike laboratory-scale craters produced by the impact of dense spheres, the diameter and depth do not follow a 1/4-power-law scaling with energy, rather the exponent observed herein is approximately 0.30, as has also been found in large-scale events. Regarding the ejecta curtain of grains, its expansion obeys the same time dependence followed by shock waves produced by underground explosions. Finally, from experiments in a two-dimensional system, the early cavity growth is analyzed and compared to a recent study on explosions at the surface of water.

Penetration in bimodal, polydisperse granular material.

We investigate the impact penetration of spheres into granular media which are compositions of two discrete size ranges, thus creating a polydisperse bimodal material. We examine the penetration depth as a function of the composition (volume fractions of the respective sizes) and impact speed. Penetration depths were found to vary between δ=0.5D_{0} and δ=7D_{0}, which, for mono-modal media only, could be correlated in terms of the total drop height, H=h+δ, as in previous studies, by incorporating correction factors for the packing fraction. Bimodal data can only be collapsed by deriving a critical packing fraction for each mass fraction. The data for the mixed grains exhibit a surprising lubricating effect, which was most significant when the finest grains [d_{s}∼O(30) µm] were added to the larger particles [d_{l}∼O(200-500) µm], with a size ratio, ε=d_{l}/d_{s}, larger than 3 and mass fractions over 25%, despite the increased packing fraction. We postulate that the small grains get between the large grains and reduce their intergrain friction, only when their mass fraction is sufficiently large to prevent them from simply rattling in the voids between the large particles. This is supported by our experimental observations of the largest lubrication effect produced by adding small glass beads to a bed of large sand particles with rough surfaces.

Spreading, encapsulation and transition to arrested shapes during drop impact onto hydrophobic powders.

We present findings from an experimental study of the impact of liquid droplets onto powder surfaces, where the particulates are hydrophobic. We vary both the size of the drop and impact speed coupled with the size range of the powder in order to assess the critical conditions for the formation of liquid marbles, where the drop becomes completely encapsulated by the powder, and arrested shapes where the drop cannot regain its spherical shape. By using different hydrophobization agents we find that a lower particle mobility may aid in promoting liquid marble formation at lower impact kinetic energies. From observations of the arrested shape formations, we propose that simple surface tensions may be inadequate to describe deformation dynamics in liquid marbles.

Impact of granular drops.

We investigate the spreading and splashing of granular drops during impact with a solid target. The granular drops are formed from roughly spherical balls of sand mixed with water, which is used as a binder to hold the ball together during free-fall. We measure the instantaneous spread diameter for different impact speeds and find that the normalized spread diameter d/D grows as (tV/D)(1/2). The speeds of the grains ejected during the "splash" are measured and they rarely exceed twice that of the impact speed.

Leaping shampoo glides on a lubricating air layer.

When a stream of shampoo is fed onto a pool in one's hand, a jet can leap sideways or rebound from the liquid surface in an intriguing phenomenon known as the Kaye effect. Earlier studies have debated whether non-Newtonian effects are the underlying cause of this phenomenon, making the jet glide on top of a shear-thinning liquid layer, or whether an entrained air layer is responsible. Herein we show unambiguously that the jet slides on a lubricating air layer. We identify this layer by looking through the pool liquid and observing its rupture into fine bubbles. The resulting microbubble sizes suggest this air layer is of submicron thickness. This thickness estimate is also supported by the tangential deceleration of the jet during the rebounding.

Sphere impact and penetration into wet sand.

We present experimental results for the penetration of a solid sphere when released onto wet sand. We show, by measuring the final penetration depth, that the cohesion induced by the water can result in either a deeper or shallower penetration for a given release height compared to dry granular material. Thus the presence of water can either lubricate or stiffen the granular material. By assuming the shear rate is proportional to the impact velocity and using the depth-averaged stopping force in calculating the shear stress, we derive effective viscosities for the wet granular materials.