David S. Cannell

Bio: David S. Cannell is an academic researcher from University of California, Santa Barbara. The author has contributed to research in topics: Convection & Light scattering. The author has an hindex of 40, co-authored 128 publications receiving 5921 citations. Previous affiliations of David S. Cannell include Memorial University of Newfoundland.

Spinodal decomposition in a binary fluid.

Abstract: Light scattering has been used to study a mixture of 3-methylpentane and nitroethane undergoing spinodal decomposition. The scattered intensity has been measured in absolute units allowing the first test or theory with no adjustable parameters. The measured structure factor does not evolve according to theory even when the initial fluctuations are taken into account. We believe the reason is that the evolving concentration field causes the sample temperature to change continuously throughout the decomposition process

Different convection dynamics in mixtures with the same separation ratio.

Abstract: We report on convection near threshold in circular samples of radial aspect ratio {Gamma}=11.5 using ethanol-water mixtures with two concentrations {ital x}, but with the same separation ratio {Psi}{approx_equal}{minus}0.08. For {ital x}=0.250, convection begins with transients that lead to a time-independent cell-filling state, typically in a day. For {ital x}=0.011, there is a range of temperature differences where repeated transients and aperiodically fluctuating localized regions of disordered convection persist for many days. In rare cases these lead to an apparently stable {open_quote}{open_quote}wall{close_quote}{close_quote} state of traveling waves surrounding pure conduction. {copyright} {ital 1996 The American Physical Society.}

Rayleigh-Bénard convection in binary mixtures with separation ratios near zero.

Abstract: We present an experimental study of convection in binary mixtures with separation ratios {Psi} close to zero. Measurements of the Hopf frequency for {Psi}{lt}0 were used to determine the relationship between {Psi} and the mass concentration {ital x} with high precision. These results are consistent with but more precise than earlier measurements by conventional techniques. For {Psi}{gt}0, we found that the pattern close to onset consisted of squares. Our data give the threshold of convection {ital r}{sub {ital c}}{equivalent_to}{ital R}{sub {ital c}}/{ital R}{sub {ital c}0} ({ital R}{sub {ital c}} is the critical Rayleigh number of the mixture and {ital R}{sub {ital c}0} that of the pure fluid) from measurements of the refractive-index power of the pattern as revealed by a very sensitive quantitative shadowgraph method. Over the range {Psi}{approx_lt}0.011, corresponding to {ital r}{sub {ital c}}{approx_gt}0.2, these results are in good agreement with linear stability analysis. The measured refractive-index power varies by six orders of magnitude as a function of {ital r} and for {ital r}{approx_gt}0.55 is in reasonable agreement with predictions based on the ten-mode Lorenz-like Galerkin truncation of Mueller and Luecke [H. W. Mueller and M. Luecke, Phys. Rev. A 38, 2965 (1988)]. For smaller {ital r}, the modelmore » predicts a cancellation between contributions to the refractive index from concentration and temperature variations, which does not seem to occur in the physical system. Determinations of the wave numbers of the patterns near onset are consistent with the theoretically predicted small critical wave numbers at positive {Psi}. As {ital r} approaches one, we find that {ital q} approaches the critical wave number {ital q}{sub {ital c}0}{congruent}3 of the pure fluid. (c) 1995 The American Physical Society« less

Accurate method for the simultaneous determination of the thermal conductivity and specific heat of nonconducting solids

Abstract: A relatively simple technique employing a two‐dimensional form of the temperature wave method is described. With reasonable care, both the specific heat and the thermal conductivity may be measured with an accuracy of 1% and a precision of 0.1%. Samples as small as 5×7 mm may readily be studied. The region of the sample in which the measurements are made is quite small and may be maintained isothermal to better than 15 mK, making the method suitable for use near phase transitions. The method should also be useful in applied fields and at high pressures or temperatures.

Pattern formation outside of equilibrium

Abstract: A comprehensive review of spatiotemporal pattern formation in systems driven away from equilibrium is presented, with emphasis on comparisons between theory and quantitative experiments. Examples include patterns in hydrodynamic systems such as thermal convection in pure fluids and binary mixtures, Taylor-Couette flow, parametric-wave instabilities, as well as patterns in solidification fronts, nonlinear optics, oscillatory chemical reactions and excitable biological media. The theoretical starting point is usually a set of deterministic equations of motion, typically in the form of nonlinear partial differential equations. These are sometimes supplemented by stochastic terms representing thermal or instrumental noise, but for macroscopic systems and carefully designed experiments the stochastic forces are often negligible. An aim of theory is to describe solutions of the deterministic equations that are likely to be reached starting from typical initial conditions and to persist at long times. A unified description is developed, based on the linear instabilities of a homogeneous state, which leads naturally to a classification of patterns in terms of the characteristic wave vector q0 and frequency ω0 of the instability. Type Is systems (ω0=0, q0≠0) are stationary in time and periodic in space; type IIIo systems (ω0≠0, q0=0) are periodic in time and uniform in space; and type Io systems (ω0≠0, q0≠0) are periodic in both space and time. Near a continuous (or supercritical) instability, the dynamics may be accurately described via "amplitude equations," whose form is universal for each type of instability. The specifics of each system enter only through the nonuniversal coefficients. Far from the instability threshold a different universal description known as the "phase equation" may be derived, but it is restricted to slow distortions of an ideal pattern. For many systems appropriate starting equations are either not known or too complicated to analyze conveniently. It is thus useful to introduce phenomenological order-parameter models, which lead to the correct amplitude equations near threshold, and which may be solved analytically or numerically in the nonlinear regime away from the instability. The above theoretical methods are useful in analyzing "real pattern effects" such as the influence of external boundaries, or the formation and dynamics of defects in ideal structures. An important element in nonequilibrium systems is the appearance of deterministic chaos. A greal deal is known about systems with a small number of degrees of freedom displaying "temporal chaos," where the structure of the phase space can be analyzed in detail. For spatially extended systems with many degrees of freedom, on the other hand, one is dealing with spatiotemporal chaos and appropriate methods of analysis need to be developed. In addition to the general features of nonequilibrium pattern formation discussed above, detailed reviews of theoretical and experimental work on many specific systems are presented. These include Rayleigh-Benard convection in a pure fluid, convection in binary-fluid mixtures, electrohydrodynamic convection in nematic liquid crystals, Taylor-Couette flow between rotating cylinders, parametric surface waves, patterns in certain open flow systems, oscillatory chemical reactions, static and dynamic patterns in biological media, crystallization fronts, and patterns in nonlinear optics. A concluding section summarizes what has and has not been accomplished, and attempts to assess the prospects for the future.

Single-molecule force spectroscopy: optical tweezers, magnetic tweezers and atomic force microscopy

Abstract: Single-molecule force spectroscopy has emerged as a powerful tool to investigate the forces and motions associated with biological molecules and enzymatic activity. The most common force spectroscopy techniques are optical tweezers, magnetic tweezers and atomic force microscopy. Here we describe these techniques and illustrate them with examples highlighting current capabilities and limitations.

On the theory of superconductivity

Abstract: IN two previous notes1, Prof. Max Born and I have shown that one can obtain a theory of superconductivity by taking account of the fact that the interaction of the electrons with the ionic lattice is appreciable only near the boundaries of Brillouin zones, and particularly strong near the corners of these. This leads to the criterion that the metal should be superconductive if a set of corners of a Brillouin zone is lying very near the Fermi surface, considered as a sphere, which limits the region in the momentum space completely filled with electrons.

Local and global instabilities in spatially developing flows

Abstract: The goal of this survey is to review recent developments in the hydro­ dynamic stability theory of spatially developing flows pertaining to absolute/convective and local/global instability concepts. We wish to dem­ onstrate how these notions can be used effectively to obtain a qualitative and quantitative description of the spatio-temporal dynamics of open shear flows, such as mixing layers, jets, wakes, boundary layers, plane Poiseuille flow, etc. In this review, we only consider open flows where fluid particles do not remain within the physical domain of interest but are advected through downstream flow boundaries. Thus, for the most part, flows in "boxes" (Rayleigh-Benard convection in finite-size cells, Taylor-Couette flow between concentric rotating cylinders, etc.) are not discussed. Further­ more, the implications of local/global and absolute/convective instability concepts for geophysical flows are only alluded to briefly. In many of the flows of interest here, the mean-velocity profile is non-