Justin Burton

Geometry of the vapor layer under a leidenfrost drop.

In the Leidenfrost effect, liquid drops deposited on a hot surface levitate on a thin vapor cushion fed by evaporation of the liquid. This vapor layer forms a concave depression in the drop interface. Using laser-light interference coupled to high-speed imaging, we measured the radius, curvature, and height of the vapor pocket, as well as nonaxisymmetric fluctuations of the interface for water drops at different temperatures. The geometry of the vapor pocket depends primarily on the drop size and not on the substrate temperature.

The inexorable resistance of inertia determines the initial regime of drop coalescence

Drop coalescence is central to diverse processes involving dispersions of drops in industrial, engineering, and scientific realms. During coalescence, two drops first touch and then merge as the liquid neck connecting them grows from initially microscopic scales to a size comparable to the drop diameters. The curvature of the interface is infinite at the point where the drops first make contact, and the flows that ensue as the two drops coalesce are intimately coupled to this singularity in the dynamics. Conventionally, this process has been thought to have just two dynamical regimes: a viscous and an inertial regime with a cross-over region between them. We use experiments and simulations to reveal that a third regime, one that describes the initial dynamics of coalescence for all drop viscosities, has been missed. An argument based on force balance allows the construction of a new coalescence phase diagram.

Coalescence of bubbles and drops in an outer fluid

When two liquid drops touch, a microscopic connecting liquid bridge forms and rapidly grows as the two drops merge into one. Whereas coalescence has been thoroughly studied when drops coalesce in vacuum or air, many important situations involve coalescence in a dense surrounding fluid, such as oil coalescence in brine. Here we study the merging of gas bubbles and liquid drops in an external fluid. Our data indicate that the flows occur over much larger length scales in the outer fluid than inside the drops themselves. Thus, we find that the asymptotic early regime is always dominated by the viscosity of the drops, independent of the external fluid. A phase diagram showing the crossovers into the different possible late-time dynamics identifies a dimensionless number that signifies when the external viscosity can be important.

Experimental and Numerical Investigation of the Equilibrium Geometry of Liquid Lenses

The equilibrium configuration of a nonwetted three fluid system takes the form of a floating liquid lens, where the lens resides between an upper and lower phase. The axisymmetric profiles of the three interfaces can be computed by solving the nonlinear Young-Laplace differential equation for each interface with coupled boundary conditions at the contact line. Here we describe a numerical method applicable to sessile or pendant lenses and provide a free, downloadable Mathematica Player file which uses a graphical interface for analyzing and plotting lens profiles. The results of the calculations were compared to optical photographs of various liquid lens systems which were analyzed using basic ray-tracing and Moiré imaging. The lens profile calculator, together with a measurement of the lens radius for a known volume, provides a simple and convenient method of determining the spreading coefficient (S) of a liquid lens system if all other fluid parameters are known. If surfactants are present, the subphase surface tension must also be self-consistently determined. A procedure is described for extracting characteristic features in the optical images to uniquely determine both parameters. The method gave good agreement with literature values for pure fluids such as alkanes on water and also for systems with a surfactant (hexadecane/DTAB), which show a transition from partial wetting to the pseudopartial wetting regime. Our technique is the analog of axisymmetric drop shape analysis, applied to a three fluid system.

A computational investigation of iceberg capsize as a driver of explosive ice-shelf disintegration

Abstract Potential energy released from the capsize of ice-shelf fragments (icebergs) is the immediate driver of the brief explosive phase of ice-shelf disintegration along the Antarctic Peninsula (e.g. the Larsen A, Larsen B and Wilkins ice shelves). The majority of this energy powers the rapidly expanding plume of ice-shelf fragments that expands outward into the open ocean; a smaller fraction of this energy goes into surface gravity waves and other dynamic interactions between ice and water that can sustain the continued fragmentation and break-up of the original ice shelf. As an initial approach to the investigation of ice-shelf fragment capsize in ice-shelf collapse, we develop a simple conceptual model involving ideal rectangular icebergs, initially in unstable or metastable orientations, which are assembled into a tightly packed mass that subsequently disassembles via massed capsize. Computations based on this conceptual model display phenomenological similarity to aspects of real ice-shelf collapse. A promising result of the conceptual model presented here is a description of how iceberg aspect ratio and its statistical variance, the two parameters related to ice-shelf fracture patterns, influence the enabling conditions to be satisfied by slow-acting processes (e.g. environmentally driven melting) that facilitate ice-shelf disintegration.

Reverse glacier motion during iceberg calving and the cause of glacial earthquakes

Movers and shakers When the edge of an ice sheet breaks off and falls into the sea (calves), the remaining section of the ice sheet moves backward and down and can suffer a glacial earthquake. Murray et al. studied calving from Greenlands Helheim Glacier. The forces that cause the change in the motion of the ice sheet at its terminus also trigger the accompanying earthquakes. Because these seismic signals can be detected by instruments located all over the globe, it should be possible to use these glacial earthquakes as proxies for glacier calving. Science, this issue p. 305 Iceberg calving causes glacial earthquakes and reverses ice sheet motion. Nearly half of Greenland’s mass loss occurs through iceberg calving, but the physical mechanisms operating during calving are poorly known and in situ observations are sparse. We show that calving at Greenland’s Helheim Glacier causes a minutes-long reversal of the glacier’s horizontal flow and a downward deflection of its terminus. The reverse motion results from the horizontal force caused by iceberg capsize and acceleration away from the glacier front. The downward motion results from a hydrodynamic pressure drop behind the capsizing berg, which also causes an upward force on the solid Earth. These forces are the source of glacial earthquakes, globally detectable seismic events whose proper interpretation will allow remote sensing of calving processes occurring at increasing numbers of outlet glaciers in Greenland and Antarctica.

Simulations of coulombic fission of charged inviscid drops.

We present boundary-integral simulations of the evolution of critically charged droplets. For such droplets, small perturbations are unstable and eventually lead to the formation of a lemon-shaped drop with very sharp tips. For perfectly conducting drops, the tip forms a self-similar cone shape with a subtended angle identical to that of a Taylor cone, and quantities such as pressure and velocity diverge in time with power-law scaling. In contrast, when charge transport is described by a finite conductivity, we find that small progeny drops are formed at the tips, whose size decreases as the conductivity is increased. These small progeny drops are of nearly critical charge, and are precursors to the emission of a sustained flow of liquid from the tips as observed in experiments of isolated charged drops.

Two-dimensional inviscid pinch-off: An example of self-similarity of the second kind

The pinch-off of a two-dimensional region of inviscid fluid is investigated using numerical and analytical techniques. We find that pinch-off occurs when a sufficiently deformed 2D drop is released from rest. The asymptotic collapse of the pinching region is characterized by an anomalous, nonrational similarity exponent α, indicating the existence of self-similarity of the second kind. Numerical solutions of the boundary integral equations show that the height of the pinch region shrinks faster than the width, so that the singularity can be described by a slender approximation. The partial differential equations obtained from this approximation are solved and are consistent with the full boundary integral methods. Furthermore, by casting the partial differential equations into similarity form, we solve a nonlinear eigenvalue problem to obtain the value of the similarity exponent, α=0.6869±0.0003.

Impact of hydrodynamics on seismic signals generated by iceberg collisions

Abstract Full-glacier-thickness icebergs are frequently observed to capsize as they calve into the ocean. As they capsize they may collide with the glaciers’ termini; previous studies have hypothesized that such collisions are the source of teleseismic ‘glacial earthquakes’. We use laboratory-scale experiments, force-balance modeling and theoretical arguments to show that (1) the contact forces during these collisions are strongly influenced by hydrodynamic forces and (2) the associated glacial earthquake magnitudes (expressed as twice-integrated force histories) are related to the energy released by the capsizing icebergs plus a hydrodynamic term that is composed of drag forces and hydrodynamic pressure. Our experiments and first-order modeling efforts suggest that, due to hydrodynamic forces, both contact force and glacial earthquake magnitudes may not be directly proportional to the energy released by the capsizing icebergs (as might be expected). Most importantly, however, our results highlight the need to better understand the hydrodynamics of iceberg capsize prior to being able to accurately interpret seismic signals generated by iceberg collisions.

Cryogenic vacuum tribology of diamond and diamond-like carbon films

Friction measurements have been performed on microcrystalline, ultrananocrystalline, and diamond-like carbon (DLC) films with natural diamond counterfaces in the temperature range of 8 K to room temperature. All films exhibit low friction (μ≤0.1) in air at room temperature. In ultrahigh vacuum, microcrystalline diamond quickly wears into a high friction state (μ≈0.6), which is independent of temperature. DLC has low friction even at the lowest temperatures. In contrast, friction in ultrananocrystalline films has a significant temperature dependence, with a broad transition from a low to a high friction state between 120 and 220 K observed on both heating and cooling. The role of hydrogen transport in determining the temperature dependence of friction is discussed.