Biomedical subjects

Michael P Brenner

Publications and source records attributed to Michael P Brenner.

10 recordsLinked to original sources

PubMed2006

Slip, yield, and bands in colloidal crystals under oscillatory shear.

We study dense colloidal crystals under oscillatory shear using a confocal microscope. At large strains the crystals yield and the suspensions form shear bands. The pure harmonic response exhibited by the suspension rules out the applicability of nonlinear rheology models typically used to describe shear banding in other types of complex fluids. Instead, we show that a model based on the coexistence of linearly responding phases of the colloidal suspension accounts for the observed flows. These results highlight a new use of oscillatory measurements in distinguishing the contribution of linear and nonlinear local rheology to a globally nonlinear material response.

Journal Article↗

PubMed2006

Self-assembly of spherical particles on an evaporating sessile droplet.

Particles adsorbed on the surface of a droplet form three-dimensional packings when the droplet evaporates. We study the final packings when the liquid droplet is attached to a solid substrate. In contrast to a droplet evaporating away from a substrate, here the final packings are highly dependent on both the number of particles and the contact angle between the droplet and the surface. Simple geometrical constraints quantitatively determine the parameter regions that particular packings can form.

Journal Article↗

PubMed2005

Shocks in ion sputtering sharpen steep surface features.

We report a regime of ion beam sputtering that occurs for sufficiently steep slopes. High slopes propagate over large distances without dissipating the steepest features. Both the propagation velocity and the dynamically selected slope are universal, independent of the details of the initial shape of the surface. The resulting behavior can be understood as the propagation of a shock front that self-selects a stable slope, as has been previously observed in thin-film fluid flows. Experiments confirm predictions of the theory. An important implication of the propagative behavior at high surface slopes is that a pattern can be fabricated at a large length scale and, through uniform ion irradiation, reduced to a smaller length scale while preserving, or even sharpening, the sharpest features.

Journal Article↗

PubMed2004

Evaporation-driven assembly of colloidal particles.

Colloidal particles absorbed at the interface of a liquid droplet arrange into unique packings during slow evaporation [V.N. Manoharan et al., Science 301, 483 (2003)]. We present a numerical and theoretical analysis of the packing selection problem. The selection of a unique packing arises almost entirely from geometrical constraints during the drying.

Biophysics↗

PubMed2004

Dynamic mechanisms for apparent slip on hydrophobic surfaces.

Recent experiments [Y. Zhu and S. Granick, Phys. Rev. Lett. 87, 096105 (2001)] have measured a large, shear-dependent fluid slip at partially wetting fluid-solid surfaces. We present a simple model for such a slip, motivated by the recent observations of nanobubbles on hydrophobic surfaces. The model considers the dynamic response of bubbles to change in hydrodynamic pressure, due to the oscillation of a solid surface. Both the compression and diffusion of gas in the bubbles decrease the force on the oscillating surface by a "leaking mattress" effect, thereby creating an apparent shear-dependent slip. With bubbles similar to those observed by recent atomic force microscopy, the model predicts a force decrease consistent with the experimental measurements of Zhu and Granick.

Journal Article↗

PubMed2004

The optimal faucet.

The production of small fluid droplets relies on an instability of solutions to the Young-Laplace equation. We ask whether smaller droplets can be produced by changing the shape of the nozzle. At a given critical pressure, the circular nozzle actually produces the largest droplet. The droplet volume can be decreased by up to 18% using a triangular nozzle with stretched corners.

Journal Article↗

PubMed2004

Elastic instability in growing yeast colonies.

The differential adhesion between cells is believed to be the major driving force behind the formation of tissues. The idea is that an aggregate of cells minimizes the overall adhesive energy between cell surfaces. We demonstrate in a model experimental system that there exist conditions where a slowly growing tissue does not minimize this adhesive energy. A mathematical model demonstrates that the instability of a spherical shape is caused by the competition between elastic and surface energies.

Adhesiveness↗

PubMed2003

Motility of Escherichia coli cells in clusters formed by chemotactic aggregation.

Cells of Escherichia coli under conditions of certain cellular stresses excrete attractants. Cells of chemotactic strains respond to these excreted signaling molecules by moving up their local concentration gradients and forming different types of stable multicellular structures. Multicellular clusters are the simplest among these structures. Fluorescence microscopy was used to characterize the macroscopic properties of the clusters and to track individual E. coli cells in the clusters in real time. A quantitative analysis reveals that the equilibrium cluster size is only weakly dependent on the total number of cells in the cluster. The tumble frequency of an individual cell strongly depends on the position of the cell within the cluster and its direction of movement. In the central region of the cluster, tumbles are strongly suppressed whereas near the edge of the cluster, the tumble frequency is restored for exiting cells, thereby preventing them from leaving the cluster, resulting in the maintenance of sharp cluster boundaries. A simulation based on a model of the sensory memory of E. coli reproduces the experimental data and indicates that the tumble rate and consequently the morphology of the cluster are determined by the sensory memory of cells.

Biophysical Phenomena↗

PubMed2003

Optimal design of a bistable switch.

Determining optimally designed structures is important for diverse fields of science and engineering. Here we describe a procedure for calculating the optimal design of a switch and apply the method to a bistable microelectromechanical system relay switch. The approach focuses on characterizing the unstable transition state connecting the two stable equilibria to control the force displacements. Small modifications in component shape lead to a substantial improvement in device operation. Fabrication of the optimized devices confirms the predictions.

Journal Article↗

PubMed2003

Controlling the fiber diameter during electrospinning.

We present a simple analytical model for the forces that determine jet diameter during electrospinning as a function of surface tension, flow rate, and electric current in the jet. The model predicts the existence of a terminal jet diameter, beyond which further thinning of the jet due to growth of the whipping instability does not occur. Experimental data for various electrospun fibers attest to the accuracy of the model.

Journal Article↗