Grain boundary melting in ice.

We describe an optical scattering study of grain boundary premelting in water ice. Ubiquitous long ranged attractive polarization forces act to suppress grain boundary melting whereas repulsive forces originating in screened Coulomb interactions and classical colligative effects enhance it. The liquid enhancing effects can be manipulated by adding dopant ions to the system. For all measured grain boundaries this leads to increasing premelted film thickness with increasing electrolyte concentration. Although we understand that the interfacial surface charge densities q(s) and solute concentrations can potentially dominate the film thickness, we cannot directly measure them within a given grain boundary. Therefore, as a framework for interpreting the data we consider two appropriate q(s) dependent limits; one is dominated by the colligative effect and other is dominated by electrostatic interactions.

A direct optical method for the study of grain boundary melting.

The structure and evolution of grain boundaries underlies the nature of polycrystalline materials. Here we describe an experimental apparatus and light reflection technique for measuring disorder at grain boundaries in optically clear material, in thermodynamic equilibrium. The approach is demonstrated on ice bicrystals. Crystallographic orientation is measured for each ice sample. The type and concentration of impurity in the liquid can be controlled and the temperature can be continuously recorded and controlled over a range near the melting point. The general methodology is appropriate for a wide variety of materials.

Light scattering from an isotropic layer between uniaxial crystals.

We develop a model for the reflection and transmission of plane waves by an isotropic layer sandwiched between two uniaxial crystals of arbitrary orientation. In the laboratory frame, reflection and transmission coefficients corresponding to the principal polarization directions in each crystal are given explicitly in terms of the [Formula: see text] axis and propagation directions. The solution is found by first deriving explicit expressions for reflection and transmission amplitude coefficients for waves propagating from an arbitrarily oriented uniaxial anisotropic material into an isotropic material. By combining these results with Lekner's (1991 J. Phys.: Condens. Matter3 6121-33) earlier treatment of waves propagating from isotropic media to anisotropic media and employing a matrix method we determine a solution to the general form of the multiple reflection case. The example system of a wetted interface between two ice crystals is used to contextualize the results.

Development, principles, and applications of automated ice fabric analyzers.

We review the recent development of automated techniques to determine the fabric and texture of polycrystalline ice. The motivation for the study of ice fabric is first outlined. After a brief introduction to the relevant optical concepts, the classic manual technique for fabric measurement is described, along with early attempts at partial automation. Then, the general principles behind fully automated techniques are discussed. We describe in some detail the similarities and differences of the three modern instruments recently developed for ice fabric studies. Next, we discuss briefly X-ray, radar, and acoustic techniques for ice fabric characterization. We also discuss the principles behind automated optical techniques to measure fabric in quartz rock samples. Finally, examples of new applications that have been facilitated by the development of the ice fabric instruments are presented.

Oscillatory flow in jet pumps: nonlinear effects and minor losses.

A nonresonant, lumped-element technique is used to investigate the behavior of tapered cylindrical flow constrictions (jet pumps) in the nonlinear oscillatory flow regime. The array of samples studied spans a wide range of inlet curvature radii and taper angles. By measuring the rectified steady pressure component developed across a jet pump as well as the acoustic impedance, the minor loss coefficients for flow into and out of the narrow end of the jet pump are determined. These coefficients are found to be relatively insensitive to all but the smallest curvature radii (i.e., sharp edges). For fixed radius of curvature, the inflow minor loss coefficient increases with increasing taper angle while the outflow coefficient remains relatively constant. For all of the samples, the steady flow minor loss coefficients are also measured and compared to their oscillatory flow counterparts. The agreement is good, confirming the so-called Iguchi hypothesis.

Lumped-element technique for the measurement of complex density.

A novel lumped-element technique is employed to measure the complex density of a gas in circular pores. The complex density expresses the geometry-dependent viscous coupling between the gas and the pore walls and is related to the thermoacoustic function f(mu), or equivalently, F(lambdaV). The acoustic impedance of a compliant region coupled to a pore (or pore-array) is measured and the impedance of the compliant region is subtracted to yield the impedance of the pore(s) alone, which is directly related to F(lambdaV). Pores of different lengths are measured in order to eliminate end effects. Working down to very low frequencies achieves a wide range of values for the ratio of the viscous penetration depth to the mean pore size. The results agree very well with analytical solutions for circular pores. The technique is also applied to two porous foam materials. Comparing the results to previous measurements of the complex compressibility, it is shown that two different shape factors (or equivalently, characteristic dimensions) are required to account for the data.

High-amplitude thermoacoustic effects in a single pore.

Nonlinear effects on thermoacoustic gain in a single pore are investigated experimentally. By creating a sharp temperature gradient in a uniform cross-section pore, the effect of high displacement amplitudes relative to the stack length is isolated from other high amplitude effects and also from effects due to geometrical discontinuities. The experiment probes displacement amplitudes which lie beyond the range of validity of the linear theory. The complex compressibility of nitrogen gas in the pore is measured for displacement amplitudes ranging from 2.5% to 60% of the stack length. No changes in the thermoacoustic response are observed over this range. Extending the upper limit to 175%, the power flow, as a function of the squared ratio of the displacement amplitude and the stack length, behaves linearly over the entire range.