P. Taborek

The PCP Pathway Instructs the Planar Orientation of Ciliated Cells in the Xenopus Larval Skin

Planar cell polarity (PCP) is a property of epithelial tissues where cellular structures coordinately orient along a two-dimensional plane lying orthogonal to the axis of apical-basal polarity. PCP is particularly striking in tissues where multiciliate cells generate a directed fluid flow, as seen, for example, in the ciliated epithelia lining the respiratory airways or the ventricles of the brain. To produce directed flow, ciliated cells orient along a common planar axis in a direction set by tissue patterning, but how this is achieved in any ciliated epithelium is unknown. Here, we show that the planar orientation of Xenopus multiciliate cells is disrupted when components in the PCP-signaling pathway are altered non-cell-autonomously. We also show that wild-type ciliated cells located at a mutant clone border reorient toward cells with low Vangl2 or high Frizzled activity and away from those with high Vangl2 activity. These results indicate that the PCP pathway provides directional non-cell-autonomous cues to orient ciliated cells as they differentiate, thus playing a critical role in establishing directed ciliary flow.

Actin and microtubules drive differential aspects of planar cell polarity in multiciliated cells

Actin dynamics are required for proper cilia spacing, global coordination of cilia polarity, and coordination of metachronic cilia beating, whereas cytoplasmic microtubule dynamics are required for local coordination of polarity between neighboring cilia.

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 low drift high resolution cryogenic null ellipsometer

A cryogenic ellipsometer capable of measuring submonolayers of liquid helium has been developed. Methods for controlling drift and noise created by the cryostat windows and ellipsometer optics are discussed in detail. The cryostat vacuum windows were made from SF-57 glass due to its low stress-optic coefficient. Custom low-stress vacuum window mounts were designed and all windows were temperature controlled. Placing the compensator wave plate inside the cryostat was necessary to avoid room temperature fluctuation induced noise. These steps produced 12 h drifts in the measured polarization of less than 0.002°. A helium adsorption isotherm taken at 1.5 K on gold is presented to show the high resolution obtained once noise and drift associated with window birefringence is minimized.

Amorphous carbon films deposited from carbon ions extracted from a discharge in fullerene vapor

Amorphous carbon (a-C) films with physical properties ranging from diamond like to graphite like were deposited at rates of 1 μm/h under 10−6 Torr vacuum by extracting a molecular carbon ion beam from a pure carbon discharge in fullerene vapor. With ion energies fixed near 300 eV, substrate temperature and beam neutralization were the main process parameters controlling film properties. a-C stress and density increased with decreasing deposition temperature, ranging to greater than 3 GPa and 2.9 g/cm3, respectively. Room temperature conductivity ranged from 10 to 10−11(Ω cm)−1, with cool substrates and a well neutralized ion beam producing the most insulating films. a-C conductivity increased irreversibly after imposition of electric fields exceeding 108 V/m through the film planes. The high conductivities of films deposited without a beam neutralizer are attributed to phase changes resulting from dielectric breakdown driven by electric fields induced by surface charging. Conductivity data obtained for fi...

Experimental Survey of Wetting and Superfluid Onset of 4He on Alkali Metal Surfaces

We present a survey of the wetting behavior of4He on evaporated films of rubidium, potassium and sodium.4He wets these surfaces at all temperatures. Off coexistence on rubidium and potassium there is a prewetting transition that is closely coupled with superfluid onset and possibly a surface tricritical point where the prewetting transition and superfluid onset intersect. Sodium has a prewetting critical temperature well below 1 K. Wetting and superfluid onset phase diagrams for4He on rubidium and potassium are presented and compared with that of4He on cesium.

Adsorption of 3He on Cesium

Adsorption isotherms of3He on cesium substrates have been measured in the temperature range from 0.2 K to 1.5 K. At liquid-vapor coexistence3He wets cesium at all temperatures studied. Step-like features are found in the isotherms which are similar to the prewetting transitions of4He on Cs substrates, but the width of these steps is ∼20 times wider for3He than for4He. In the case of3He on Cs, the steps are located at a chemical potential about 0.6 K below liquid-vapor coexistence. If the low temperature behavior is interpreted to be first order prewetting, the prewetting critical point temperature is 0.6±0.1 K.

Potential role of atomic carbon in diamond deposition

This paper presents a numerical simulation of the gas‐phase chemistry in diamond deposition processes. The simulation shows that the concentration of the two potential growth species CH3 and C in the boundary layer near the diamond film substrate are sensitive to these species’ concentration in the bulk gas. The concentrations in the bulk gas depend, in turn, on the physical arrangement of the reactor, and in particular on the time provided for the gas mixture to reach chemical equilibrium. With sufficient equilibration time, simulations of both a hot‐filament reactor and a plasma torch reactors show that the concentration of atomic carbon at the substrate surface can be much higher than the concentration of CH3.

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.

Temperature dependence of single-asperity friction for a diamond on diamondlike carbon interface

A variable temperature, ultrahigh vacuum atomic force microscope (AFM) was used to characterize interfacial friction for a single-asperity diamond contact on a diamondlike carbon (DLC) substrate over a nominal substrate temperature range of 90 to 275 K. Calibrated friction force measurements were obtained by analyzing lateral force hysteresis loops as a function of normal force. For sufficiently large normal forces, the lateral force was proportional to the normal force, and a friction coefficient μ could be identified. μ varied approximately linearly with substrate temperature, with μ=0.28 at T=90 K and μ=0.38 at 275 K. These results are compared to other recent variable temperature AFM friction measurements and to theoretical calculations based on the Tomlinson model. This comparison is obscured by large, experimentally uncontrolled temperature differences between the tip and the substrate which inevitably exist in conventional, variable temperature AFMs. A thermal model which can be used to quantitativ...