Wetting transition in water.

Optical images were used to study the wetting behavior of water on graphite, sapphire, and quartz along the liquid vapor coexistence curve from room temperature to 300 °C. Wetting transitions were identified by the temperature at which the contact angle decreased to zero and also by the disappearance of dropwise condensation. These two methods yielded consistent values for the wetting temperatures, which were 185 °C, 234 °C, and 271 °C for water on quartz, sapphire, and graphite, respectively. We compare our results with the theoretical predictions based on a simplified model of the water-substrate potential and sharp interfaces.

Pressure-driven flow through a single nanopore.

We have measured the flow of gas through single ion track pores in a polymer film using a mass spectroscopy technique. The pores are 12 µm long with diameters in the range of 50-1000 nm, and the flow was driven by pressure drops in the range 0-30 atm. When the mean free path is large compared to the pore diameter (large Knudsen number Kn), the flow rate is proportional to the pressure drop and the pore radius R cubed, and is consistent with a model of diffusive scattering at the pore walls. For Kn≤0.1, the hydrodynamic conductance increases, as predicted by standard kinetic theory models, and finally approaches the conventional Poiseuille value with zero slip length.

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.

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.

Bifurcation from bubble to droplet behavior in inviscid pinch-off.

High-speed video is used to analyze the pinch-off of xenon in water. By varying the pressure in the xenon from 0.05 to 68 atm, the mass density ratio D=rho(int)/rho(ext) of the interior gaseous xenon rho(int} to the exterior liquid water rho(ext) can be varied by over 3 orders of magnitude. Both the shape of the pinch region and the power law that governs the collapse of the minimum neck radius are distinctly different for the low D (bubblelike) and high D (dropletlike) cases. We show that there is a rather abrupt transition between these types of behavior near D approximately 0.25.

Role of dimensionality and axisymmetry in fluid pinch-off and coalescence.

We present data on the pinch-off and coalescence of thin liquid alkane lenses floating on water. Pinch-off in quasi-2D lenses is distinctly different from pinch-off in axisymmetric 3D drops and involves a cascade of satellite droplets which extends to micron length scales. In contrast, coalescence of lenses is qualitatively similar to coalescence of 3D drops. Coalescence is predicted to involve entrainment of the exterior fluid as the droplets merge. This reentrant folding is obscured in 3D droplets but is clearly visible in coalescence of thin lenses.

Fluid pinch-off in superfluid and normal 4He.

We present frames from high-speed videos of the pinch-off of liquid 4He droplets. The temperature of the fluid droplets ranged from 1.33 K to 4.8 K, and the size of the drops was proportional to the temperature-dependent capillary length. We observed no qualitative difference between pinch-off in the normal and superfluid states. In both cases, the shape of the fluid in the final stages of pinch-off resembles a cone piercing a sphere, which is typical of other low-viscosity fluids. The evolution of the minimum neck radius rmin can be characterized by power laws rmin proportional, taun, where tau is the time remaining until pinch-off occurs. In the regime near pinch-off, the data from image analysis are consistent with n=2/3. The data at the beginning of the pinch process when the neck is of the order of the capillary length are also described by n=2/3, but with a different proportionality factor. There is an intermediate crossover regime characterized by n=2/5.

Scaling and instabilities in bubble pinch-off.

We have used a 100 000 frame-per-second video to analyze the pinch-off of nitrogen gas bubbles in fluids with a wide range of viscosity. If the external fluid is highly viscous (eta(ext)>100 cP), the bubble neck radius is proportional to the time before break, tau, and decreases smoothly to zero. If the external fluid has low viscosity (eta(ext)<10 cP), the radius scales as tau(1/2) until an instability develops in the gas bubble, which causes the neck to rupture and tear apart. Finally, if the viscosity of the external fluid is in an intermediate range, an elongated thread is formed, which breaks apart into micron-sized bubbles.

Fluid pinch-off dynamics at nanometer length scales.

The breakup of a drop of inviscid fluid into two smaller drops is determined by a competition between surface and inertial forces. This process forms a thin filament of fluid with a connecting neck that shrinks to zero diameter at a finite time singularity. We present measurements of the electrical resistance of a liquid bridge of mercury as it undergoes pinch off. The electrical measurements allow us to probe the region of the singularity down to nanosecond times and nanometer lengths. Near pinch off, the resistance of the liquid bridge diverges as t(-2/3), as expected for inviscid flow.