A liquid drop impacting a solid surface may splash either by emitting a thin liquid sheet that subsequently breaks apart or by promptly ejecting droplets from the advancing liquid-solid contact line. Using high-speed imaging, we show that surface roughness and air pressure influence both mechanisms. Roughness inhibits thin-sheet formation even though it also increases prompt splashing at the advancing contact line. If the air pressure is lowered, droplet ejection is suppressed not only during thin-sheet formation but for prompt splashing as well.PACS numbers: 47.20.Cq, 47.20.Gv, 47.20.Ma,Will a drop hitting a dry surface splash? Different criteria [1][2][3][4][5] have been proposed to predict when such a drop will splash by comparing the roughness of the solid surface with hydrodynamic length scales, which depend on parameters such as the drop velocity, radius, viscosity and surface tension. Several years ago Xu et al. [6,7] found that these criteria ignore a crucial parameter: the ambient gas pressure, P . When a drop splashes on a smooth surface it spreads smoothly forming a lamella before ejecting a thin sheet that subsequently breaks up into secondary droplets. As P is reduced below a threshold pressure, the drop no longer splashes [6][7][8][9][10]. On the other hand, when splashing occurs on a rough surface, no thin sheet is formed and droplets are ejected directly from the advancing liquid-substrate contact line via a "prompt" splash [1][2][3][4]8].It has been suggested that thin-sheet splashes depend on air pressure while prompt splashes do not and depend only on surface roughness [8]. Here we show that the situation is more complex in that both types of splashing depend, albeit in opposite ways, on surface roughness. In particular, we observe four distinct regimes. In agreement with earlier results [4], we observe a thin-sheet splash on very smooth surfaces and a prompt splash on very rough ones. However, at intermediate roughness, we identify two new regimes: at low viscosities both prompt and thin-sheet splashes occur during a single impact, while at high viscosities neither splash is formed. In addition, as found for thin-sheet splashing [6], we find that a drop deposits smoothly on a rough surface if P is low enough. Clearly, the role of both air pressure and substrate roughness must be considered in all cases.The experiments were conducted with silicone oil (PDMS, Clearco Products) with kinematic viscosity ν ranging from 5 cSt to 14.4 cSt and surface tension σ between 19.7 dyn/cm and 20.8 dyn/cm. The basic results were replicated using water/glycerin mixtures with a similar viscosity range but higher surface tension: σ=67 dyn/cm. Low-viscosity impacts were studied with ethanol. Drops with reproducible diameter D=3.1 mm were produced using a syringe pump (Razel Scientific, Model R99-E) and released in a chamber from a height above a substrate. This height set the impact velocity u 0 which was varied between 2.7 m/s and 4.1 m/s. These parameters determine the Reynolds number Re=Du 0 /ν giving the rati...

We probe the flow of two dimensional foams, consisting of a monolayer of bubbles sandwiched between a liquid bath and glass plate, as a function of driving rate, packing fraction and degree of disorder. First, we find that bidisperse, disordered foams exhibit strongly rate dependent and inhomogeneous (shear banded) velocity profiles, while monodisperse, ordered foams are also shear banded, but essentially rate independent. Second, we introduce a simple model based on balancing the averaged drag forces between the bubbles and the top plate,F bw and the averaged bubblebubble drag forcesF bb , and assume thatF bw ∼ v 2/3 andF bb ∼ (∂yv) β , where v and (∂yv) denote average bubble velocities and gradients. This model captures the observed rate dependent flows for β ≈ 0.36, and the rate independent flows for β ≈ 0.67. Third, we perform independent rheological measurements ofF bw andF bb , both for ordered and disordered systems, and find these to be fully consistent with the forms assumed in the simple model. Disorder thus modifies the exponent β. Fourth, we vary the packing fraction φ of the foam over a substantial range, and find that the flow profiles become increasingly shear banded when the foam is made wetter. Surprisingly, our model describes flow profiles and rate dependence over the whole range of packing fractions with the same power law exponents -only a dimensionless number k which measures the ratio of the pre-factors of the viscous drag laws is seen to vary with packing fraction. We find that k ∼ (φ − φc)−1 , where φc ≈ 0.84, corresponding to the 2d jamming density, and suggest that this scaling follows from the geometry of the deformed facets between bubbles in contact. Overall, our work shows that the presence of disorder qualitatively changes the effective bubble-bubble drag forces, and suggests a route to rationalize aspects of the ubiquitous Herschel-Bulkley (power law) rheology observed in a wide range of disordered materials.

We explore the evolution of a splash when a liquid drop impacts a smooth, dry surface. There are two splashing regimes that occur when the liquid viscosity is varied, as is evidenced by its dependence on ambient gas pressure. A high-viscosity drop splashes by emitting a thin sheet of liquid from a spreading liquid lamella long after the drop has first contacted the solid. Likewise, we find that there is also a delay in the ejection of a thin sheet when a low-viscosity drop splashes. We show how the ejection time of the thin sheet depends on liquid viscosity and ambient gas pressure.PACS numbers: 47.20.Gv,47.55.Ca,The discovery by Xu et al. [1], that the splash of a liquid drop hitting a smooth dry surface is suppressed by lowering the ambient air pressure, has galvanized research on gas-liquid interactions during impact. However, despite numerous experimental [2][3][4][5][6][7][8], theoretical [9,10], and numerical [3,11,12] efforts, the mechanism by which air causes a drop to splash remains unresolved.The situation is made more complicated, by the influence of liquid viscosity µ on the interplay of gas and liquid. At low viscosities, a beautiful crown-shaped corona emerges almost immediately after impact as shown in Fig. 1(a) [1,2]. However, a small increase in viscosity reveals a splash with a strikingly different appearance, that evolves much more slowly ( Fig. 1(b)). This higher-µ drop first contacts the surface and then spreads smoothly as a thick lamellar sheet. From this lamella, a thinner sheet of liquid is subsequently ejected almost parallel to the substrate. It is the thin sheet that eventually breaks apart to form the splash [4]. The existence of two distinct splashing regimes is made manifest in the non-monotonic dependence of the threshold pressure, P T , which is the ambient gas pressure above which splashing occurs, on the viscosity [2]. As shown in Fig. 2, P T decreases with increasing viscosity at low-µ, while the trend is reversed at higher µ.These differences have been taken to suggest that distinct mechanisms might underlie the two types of splash. Indeed, theories for low-µ splashes have been proposed that do not take into account any spreading of a liquid film on the substrate before the onset of the splash [9,10]. On the other hand, the fact that, regardless of viscosity, splashes are invariably suppressed when the ambient pressure is sufficiently low suggests that there may be a common mechanism for both the violent corona and the slowly evolving thin sheet. It is therefore imperative that one investigate whether the splash mechanisms in these two cases have common features even though the timescales for corona (or thin-sheet) ejection and the overall shape of the splashing drops differ dramatically. This paper studies the onset of thin-sheet and corona ejection in the two cases. As previously noted, at high-µ, thin-sheet ejection is delayed when the pressure is lowered [4]. The major conclusion from the present work is that this is also true in the low-viscosity regime. Corona ejection ...

We study colloidal particle dynamics of a model glass system using confocal and fluorescence microscopy as the sample evolves from a hard-sphere glass to a liquid with attractive interparticle interactions. The transition from hard-sphere glass to attractive liquid is induced by short-range depletion forces. The development of liquid-like structure is indicated by particle dynamics. We identify particles which exhibit substantial motional events and characterize the transition using the properties of these motional events. As samples enter the attractive liquid region, particle speed during these motional events increases by about one order of magnitude, and the particles move more cooperatively. Interestingly, colloidal particles in the attractive liquid phase do not exhibit significantly larger displacements than particles in the hard-sphere glass.

At atmospheric pressure, a drop of ethanol impacting on a solid surface produces a splash. Reducing the ambient pressure below its atmospheric value suppresses this splash. The origin of this so-called pressure effect is not well understood and this is the first study to present an in-depth comparison between various theoretical models that aim to predict splashing and simulations. In this work the pressure effect is explored numerically by resolving the Navier-Stokes equations at a 3-nm resolution. In addition to reproducing numerous experimental observations, it is found that different models all provide elements of what is observed in the simulations. The skating droplet model correctly predicts the existence and scaling of a gas film under the droplet, the lamella formation theory is able to correctly predict the scaling of the lamella ejection velocity as function of the impact velocity for liquids with different viscosity, and lastly, the dewetting theory's hypothesis of a lift force acting on the liquid sheet after ejection is consistent with our results.