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.

Multiple-scattering suppression by cross correlation

Abstract: We describe a new method for characterizing particles in turbid media by cross correlating the scattered intensity fluctuations at two nearby points in the far field. The cross-correlation function selectively emphasizes single scattering over multiple scattering. The usual dynamic light-scattering capability of inferring particle size from decay rate is thus extended to samples that are so turbid as to be visually opaque. The method relies on single-scattering speckle being physically larger than multiple-scattering speckle. With a suitable optical geometry to select nearby points in the far field or equivalently slightly different scattering wave vectors (of the same magnitude), the multiple-scattering contribution to the cross-correlation function may be reduced and in some cases rendered insignificant. Experimental results demonstrating the feasibility of this approach are presented.

Dynamics of Long-Wavelength Concentration Fluctuations in Solutions of Linear Polymers

Abstract: We report experimental results for a dynamic length scale ${\ensuremath{\xi}}_{H}$ as a function of polymer concentration $c$ for various molecular weight polystyrenes in the two solvents toluene and methyl ethyl ketone. We find that the quantity $\frac{{\ensuremath{\xi}}_{H}}{{R}_{H}}$ is a function only of the reduced concentration $\mathrm{kc}$, where ${R}_{H}$ is the hydrodynamic radius and $k$ is determined from a virial expansion of the diffusion coefficient. However, the dynamic and static length scales are not proportional.

Osmotic susceptibility and diffusion coefficient of charged bovine serum albumin

Abstract: We report measurements of the osmotic susceptibility and mutual diffusion coefficient of solutions of charged bovine serum albumin molecules made using laser light scattering. The measurements were made as a function of solution ionic strength for various macromolecular charge states ranging from −4e to −13e. We find that the osmotic susceptibility data may be interpreted quantitatively in terms of the simple Verwey–Overbeek interaction potential, provided that adequate care is taken in computing the radial distribution function. Current theoretical treatments of the dynamics, with hydrodynamic interactions included to first order in the concentration, are found to deviate significantly (≲50%) from the diffusion coefficient data. The deviations occur under all conditions of charge and ionic strength for which the susceptibility has been reduced to less than half the ideal gas value.

Critical behavior near a vanishing miscibility gap.

Abstract: We report measurements of the osmotic susceptibility and correlation length for the pseudobinary mixture of guaiacol ($o$-methoxy phenol) plus glycerol-water as a function of temperature and water content $x[x\ensuremath{\equiv}\mathrm{weight}\mathrm{of}\mathrm{water}/(\mathrm{weight}\mathrm{of}\mathrm{water}\mathrm{plus}\mathrm{weight}\mathrm{of}\mathrm{glycerol})]$. This system exhibits a limited two-phase region which vanishes for $xl{x}_{0}\ensuremath{\simeq}0.0137$. We observed a smooth transition from nearly Ising-type behavior for $x\ensuremath{\gtrsim}4{x}_{0}$ through a non-power-law regime to doubled exponents for $x\ensuremath{\simeq}{x}_{0}$ and saturating divergences with doubled exponents for $xl{x}_{0}$.

Effect of Gravity on the Dynamics of Nonequilibrium Fluctuations in a Free-Diffusion Experiment

Abstract: Diffusion is commonly believed to be a homogeneous process at the mesoscopic scale, being driven only by the random walk of fluid molecules. On the contrary, very large amplitude, long wavelength fluctuations always accompany diffusive processes. In the presence of gravity, fluctuations in a fluid containing a stabilizing gradient are affected by two different processes: diffusion, which relaxes them, and the buoyancy force, which quenches them. These phenomena affect both the overall amplitude of fluctuations and their time dependence. For the case of free diffusion, the time-correlation function of the concentration fluctuations is predicted to exhibit an exponential decay with correlation time depending on the wave vector q. For large wave vector fluctuations, diffusion dominates, and the correlation time is predicted to be 1 / (Dq2). For small wave vector fluctuations, gravitational forces have time to play a significant role, and the correlation time is predicted to be proportional to q2. The effects of gravity and diffusion are comparable for a critical wave vector q(c) determined by fluid properties and gravity. We have utilized a quantitative dynamic shadowgraph technique to obtain the temporal correlation function of a mixture of LUDOX(R) TMA and water undergoing free diffusion. This technique allows one to simultaneously measure correlation functions achieving good statistics for a number of different wave vectors in a single measurement. Wave vectors as small as 70 cm(-1) have been investigated, which is very difficult to achieve with ordinary dynamic light-scattering techniques. We present results on the transition from the diffusive decay of fluctuations to the regime in which gravity is dominant.

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-