Control of ultralow pressures: an absolute thermodynamic manometer

A method of accurately manipulating the pressure of helium gas in the range P=10^–1 to 10^–20 Torr, T<5 K is presented. The method requires only a very modest apparatus and makes use of thermodynamic measurements on the 4He–Grafoil system. A calibration curve and a pressure scale for the manometer are developed which will allow convenient application in other laboratories. An experiment which utilizes the technique is briefly disscussed

Simulations of Coulombic Fission of Charged Inviscid Drops

We present boundary-integral simulations of the evolution of critically charged droplets. For such droplets, small ellipsoidal 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. At the tip, quantities such and pressure and fluid 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.Comment: 4 pages, 3 figure

Phonon reflection at a sapphire-vacuum interface

Theoretical and experimental studies have been conducted of the transmission of phonons in sapphire and their reflection at a crystal-vacuum interface. The effects of crystal anisotropy on the reflection process are discussed in detail. The heat-pulse technique has been used to obtain high-resolution time-of-flight spectra as a function of the angle of incidence in accurately known crystallographic directions. The agreement with predictions from numerical calculations is excellent. The relevance of these observations to studies of the anomalous transmission of energy through a crystal-liquid helium interface is also discussed

Diffuse reflection of phonons and the anomalous Kapitza resistance

High-resolution time-of-flight phonon-reflection spectra in sapphire crystals are analyzed with the aid of detailed numerical calculations. It is argued that spectra from vacuum interfaces may be understood if about half the incident phonons are assumed to be reflected specularly, the remainder diffusely. A simple model of the effect on spectra of diffuse scattering is presented and compared to experimental data. We show that the specularly reflected signal is unaffected by placing helium at the interface, but that the diffuse part is transmitted almost entirely, accounting for the anomalous Kapitza resistance. It is shown that conventional calculations of phonon focusing are not suitable for interpreting our data, and a new, more correct theory of phonon focusing, aided by formal catastrophe theory, is introduced. We argue that the experimental resolution and theoretical analysis we have presented are essential for understanding phonon-reflection experiments

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

Phonon induced desorption from helium films

We have investigated the problem of desorption from thin helium films on sapphire crystals, and find evidence in favor of single phonon processes