Liquid drop splashing on smooth, rough and textured surfaces

Splashing occurs when a liquid drop hits a dry solid surface at high velocity. This paper reports experimental studies of how the splash depends on the roughness and the texture of the surfaces as well as the viscosity of the liquid. For smooth surfaces, there is a "corona" splash caused by the presence of air surrounding the drop. There are several regimes that occur as the velocity and liquid viscosity are varied. There is also a "prompt" splash that depends on the roughness and texture of the surfaces. A measurement of the size distribution of the ejected droplets is sensitive to the surface roughness. For a textured surface in which pillars are arranged in a square lattice, experiment shows that the splashing has a four-fold symmetry. The splash occurs predominantly along the diagonal directions. In this geometry, two factors affect splashing the most: the pillar height and spacing between pillars.Comment: 9 pages, 11 figure

Splash control of drop impacts with geometric targets

Drop impacts on solid and liquid surfaces exhibit complex dynamics due to the competition of inertial, viscous, and capillary forces. After impact, a liquid lamella develops and expands radially, and under certain conditions, the outer rim breaks up into an irregular arrangement of filaments and secondary droplets. We show experimentally that the lamella expansion and subsequent break up of the outer rim can be controlled by length scales that are of comparable dimension to the impacting drop diameter. Under identical impact parameters, ie. fluid properties and impact velocity, we observe unique splashing dynamics by varying the target cross-sectional geometry. These behaviors include: (i) geometrically-shaped lamellae and (ii) a transition in splashing stability, from regular to irregular splashing. We propose that regular splashes are controlled by the azimuthal perturbations imposed by the target cross-sectional geometry and that irregular splashes are governed by the fastest-growing unstable Plateau-Rayleigh mode

Drop Splashing on a Dry Smooth Surface

The corona splash due to the impact of a liquid drop on a smooth dry substrate is investigated with high speed photography. A striking phenomenon is observed: splashing can be completely suppressed by decreasing the pressure of the surrounding gas. The threshold pressure where a splash first occurs is measured as a function of the impact velocity and found to scale with the molecular weight of the gas and the viscosity of the liquid. Both experimental scaling relations support a model in which compressible effects in the gas are responsible for splashing in liquid solid impacts.Comment: 11 pages, 4 figure

Making a splash with water repellency

A 'splash' is usually heard when a solid body enters water at large velocity. This phenomena originates from the formation of an air cavity resulting from the complex transient dynamics of the free interface during the impact. The classical picture of impacts on free surfaces relies solely on fluid inertia, arguing that surface properties and viscous effects are negligible at sufficiently large velocities. In strong contrast to this large-scale hydrodynamic viewpoint, we demonstrate in this study that the wettability of the impacting body is a key factor in determining the degree of splashing. This unexpected result is illustrated in Fig.1: a large cavity is evident for an impacting hydrophobic sphere (1.b), contrasting with the hydrophilic sphere's impact under the very same conditions (1.a). This unforeseen fact is furthermore embodied in the dependence of the threshold velocity for air entrainment on the contact angle of the impacting body, as well as on the ratio between the surface tension and fluid viscosity, thereby defining a critical capillary velocity. As a paradigm, we show that superhydrophobic impacters make a big 'splash' for any impact velocity. This novel understanding provides a new perspective for impacts on free surfaces, and reveals that modifications of the detailed nature of the surface -- involving physico-chemical aspects at the nanometric scales -- provide an efficient and versatile strategy for controlling the water entry of solid bodies at high velocity.Comment: accepted for publication in Nature Physic

Vortex Matter Transition in Bi2{}_2Sr2{}_2CaCu2{}_2O8+y{}_{8+y} under Tilted Fields

Vortex phase diagram under tilted fields from the $c$ axis in Bi${}_2$Sr${}_2$CaCu${}_2$O${}_{8+y}$ is studied by local magnetization hysteresis measurements using Hall probes. When the field is applied at large angles from the $c$ axis, an anomaly ($H_p^\ast$) other than the well-known peak effect ($H_p$) are found at fields below $H_p$. The angular dependence of the field $H_p^\ast$ is nonmonotonic and clearly different from that of $H_p$ and depends on the oxygen content of the crystal. The results suggest existence of a vortex matter transition under tilted fields. Possible mechanisms of the transition are discussed.Comment: Revtex, 4 pages, some corrections are adde

Phase Transitions in a Model Anisotropic High Tc Superconductor

We carry out simulations of the anisotropic uniformly frustrated 3D XY model, as a model for vortex line fluctuations in high Tc superconductors. We compute the phase diagram as a function of temperature and anisotropy, for a fixed applied magnetic field. We find that superconducting coherence parallel to the field persists into the vortex line liquid state, and that this transition lies well below the "mean-field" cross-over from the vortex line liquid to the normal state.Comment: 23 pages + 19 ps figure

Kinetic Theory of Flux Line Hydrodynamics:LIQUID Phase with Disorder

We study the Langevin dynamics of flux lines of high--T$_c$ superconductors in the presence of random quenched pinning. The hydrodynamic theory for the densities is derived by starting with the microscopic model for the flux-line liquid. The dynamic functional is expressed as an expansion in the dynamic order parameter and the corresponding response field. We treat the model within the Gaussian approximation and calculate the dynamic structure function in the presence of pinning disorder. The disorder leads to an additive static peak proportional to the disorder strength. On length scales larger than the line static transverse wandering length and at long times, we recover the hydrodynamic results of simple frictional diffusion, with interactions additively renormalizing the relaxational rate. On shorter length and time scales line internal degrees of freedom significantly modify the dynamics by generating wavevector-dependent corrections to the density relaxation rate.Comment: 61 pages and 6 figures available upon request, plain TEX using Harvard macro

A 30-year reconstruction of the Atlantic meridional overturning circulation shows no decline

A decline in Atlantic meridional overturning circulation (AMOC) strength has been observed between 2004 and 2012 by the RAPID-MOCHA-WBTS (RAPID – Meridional Overturning Circulation and Heatflux Array – Western Boundary Time Series, hereafter RAPID array) with this weakened state of the AMOC persisting until 2017. Climate model and paleo-oceanographic research suggests that the AMOC may have been declining for decades or even centuries before this; however direct observations are sparse prior to 2004, giving only “snapshots” of the overturning circulation. Previous studies have used linear models based on upper-layer temperature anomalies to extend AMOC estimates back in time; however these ignore changes in the deep circulation that are beginning to emerge in the observations of AMOC decline. Here we develop a higher-fidelity empirical model of AMOC variability based on RAPID data and associated physically with changes in thickness of the persistent upper, intermediate, and deep water masses at 26∘ N and associated transports. We applied historical hydrographic data to the empirical model to create an AMOC time series extending from 1981 to 2016. Increasing the resolution of the observed AMOC to approximately annual shows multi-annual variability in agreement with RAPID observations and shows that the downturn between 2008 and 2012 was the weakest AMOC since the mid-1980s. However, the time series shows no overall AMOC decline as indicated by other proxies and high-resolution climate models. Our results reinforce that adequately capturing changes to the deep circulation is key to detecting any anthropogenic climate-change-related AMOC decline

The cold transit of Southern Ocean upwelling

The upwelling of deep waters in the Southern Ocean is a critical component of the climate system. The time and zonal mean dynamics of this circulation describe the upwelling of Circumpolar Deep Water and the downwelling of Antarctic Intermediate Water. The thermodynamic drivers of the circulation and their seasonal cycle play a potentially key regulatory role. Here an observationally constrained ocean model and an observation‐based seasonal climatology are analyzed from a thermodynamic perspective, to assess the diabatic processes controlling overturning in the Southern Ocean. This reveals a seasonal two‐stage cold transit in the formation of intermediate water from upwelled deep water. First, relatively warm and saline deep water is transformed into colder and fresher near‐surface winter water via wintertime mixing. Second, winter water warms to form intermediate water through summertime surface heat fluxes. The mixing‐driven pathway from deep water to winter water follows mixing lines in thermohaline coordinates indicative of nonlinear processes