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.

Neutron-scattering studies of binary mixtures in silica gels.

Abstract: We report the results of neutron-scattering experiments designed to probe structure in binary mixtures confined in silica gels over the wavelength range 60\char21{}2000 \AA{}. In what would be the one-phase region of the pure system, the scattering can be fit very well to the sum of three contributions: critical fluctuations of the mixture, preferentially adsorbed fluid, and the silica gel itself. By interpreting the adsorption as the response of the order parameter to an imposed field due to the gel, we find that the q dependence of the response is consistent with linear response theory on all length scales accessible in these experiments, although the amplitude of the response does not diverge as strongly as does the order-parameter susceptibility. In what would be the two-phase region of the pure system, we observe changes in the scattering that are consistent with the existence of large, slowly coarsening domains. The results are discussed in the context of the random-field Ising model and its magnetic realizations.

Wave-vector dependence of the initial decay rate of fluctuations in polymer solutions

Abstract: We present experimental results for the initial decay rate ${\mathrm{\ensuremath{\Gamma}}}_{1}$ of concentration fluctuations in solutions of linear polymers, for various molecular weights, concentrations, and wave vector q, in two rather different solvents. We find that under all conditions explored the dimensionless decay rate 6\ensuremath{\pi}${\mathrm{\ensuremath{\eta}}}_{0}$${\mathrm{\ensuremath{\Gamma}}}_{1}$/(${\mathrm{k}}_{\mathrm{B}{\mathrm{Tq}}^{3}}$) is a function only of the scaled wave vector q${\ensuremath{\xi}}_{\mathrm{H}}$. Here ${\mathrm{\ensuremath{\eta}}}_{0}$ is the solvent viscosity, and ${\ensuremath{\xi}}_{\mathrm{H}}$ is the length scale defined by ${\ensuremath{\xi}}_{\mathrm{H}}^{\mathrm{\ensuremath{-}}1}$\ensuremath{\equiv}${\mathrm{lim}}_{\mathrm{q}\ensuremath{\rightarrow}0}$6\ensuremath{\pi}${\mathrm{\ensuremath{\eta}}}_{0}$\ensuremath{\Gamma} 1/(${\mathrm{k}}_{\mathrm{B}}$${\mathrm{Tq}}^{2}$).

European Space Agency experiments on thermodiffusion of fluid mixtures in space

Abstract: This paper describes the European Space Agency (ESA) experiments devoted to study thermodiffusion of fluid mixtures in microgravity environment, where sedimentation and convection do not affect the mass flow induced by the Soret effect First, the experiments performed on binary mixtures in the IVIDIL and GRADFLEX experiments are described Then, further experiments on ternary mixtures and complex fluids performed in DCMIX and planned to be performed in the context of the NEUF-DIX project are presented Finally, multi-component mixtures studied in the SCCO project are detailed

Thermal convection in the presence of a first-order phase change.

Abstract: We report experimental results for convection in a thin horizontal layer of a fluid which is heated from below and which undergoes a first-order phase change. For some parameter ranges the denser layer may be stably stratified above the lighter one, as predicted by Busse and Schubert. Over various parameter ranges we observed hexagonal, inverted-hexagonal, and roll flow. Interesting interactions between two-phase convection and Rayleigh-Benard convection are observed near two codimension-two points.

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-