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A. T. Chwang

Bio: A. T. Chwang is an academic researcher from California Institute of Technology. The author has contributed to research in topics: Pressure-gradient force & Gravitational singularity. The author has an hindex of 2, co-authored 2 publications receiving 311 citations.

Papers
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Journal ArticleDOI
TL;DR: In this article, the image system for the fundamental singularities of viscous (including potential) flow is obtained in the vicinity of an infinite stationary no-slip plane boundary, where the authors obtain a far field of O(r−2) for force or rotational components parallel to the wall, whereas normal components are of higher order O(ρ−3).
Abstract: The image system for the fundamental singularities of viscous (including potential) flow are obtained in the vicinity of an infinite stationary no-slip plane boundary. The image system for a: stokeslet, the fundamental singularity of Stokes flow; rotlet (also called a stresslet), the fundamental singularity of rotational motion; a source, the fundamental singularity of potential flow and also the image system for a source-doublet are discussed in terms of illustrative diagrams. Their far-fields are obtained and interpreted in terms of singularities. Both the stokeslet and rotlet have similar far field characteristics: for force or rotational components parallel to the wall a far-field of a stresslet typeO(r −2) is obtained, whereas normal components are of higher orderO(r −3).

324 citations

Journal ArticleDOI
TL;DR: In this paper, the effect of finite reservoir on the hydrodynamic pressure due to horizontal as well as vertical ground excitations has been studied and it is found that for horizontal accelerations the hydrogynamic pressure force decreases as the size of the reservoir decreases.
Abstract: The effect of finite reservoir on the hydrodynamic pressure due to horizontal as well as vertical ground excitations has been studied. It is found that for horizontal accelerations the hydrodynamic pressure force decreases as the size of the reservoir decreases. The effect of vertical acceleration on the pressure force on a dam is simply to adjust the hydrostatic pressure by replacing the gravitational constant by an effective gravitational acceleration and this is true for any arbitrary shapes of the reservoir. A simple criterion has been presented in this paper which would enable dam engineers to determine whether a given earthquake could cause cavitation at the dam-water interface or not.

11 citations


Cited by
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Journal ArticleDOI
TL;DR: The biophysical and mechanical principles of locomotion at the small scales relevant to cell swimming, tens of micrometers and below are reviewed, with emphasis on the simple physical picture and fundamental flow physics phenomena in this regime.
Abstract: Cell motility in viscous fluids is ubiquitous and affects many biological processes, including reproduction, infection and the marine life ecosystem. Here we review the biophysical and mechanical principles of locomotion at the small scales relevant to cell swimming, tens of micrometers and below. At this scale, inertia is unimportant and the Reynolds number is small. Our emphasis is on the simple physical picture and fundamental flow physics phenomena in this regime. We first give a brief overview of the mechanisms for swimming motility, and of the basic properties of flows at low Reynolds number, paying special attention to aspects most relevant for swimming such as resistance matrices for solid bodies, flow singularities and kinematic requirements for net translation. Then we review classical theoretical work on cell motility, in particular early calculations of swimming kinematics with prescribed stroke and the application of resistive force theory and slender-body theory to flagellar locomotion. After examining the physical means by which flagella are actuated, we outline areas of active research, including hydrodynamic interactions, biological locomotion in complex fluids, the design of small-scale artificial swimmers and the optimization of locomotion strategies. (Some figures in this article are in colour only in the electronic version) This article was invited by Christoph Schmidt.

2,274 citations

Journal ArticleDOI
TL;DR: This review restricts this review primarily to a summary of present understanding of the low-Reynolds-number flows associated with microorganism propulsion and the hydromechanics of ciliary systems.
Abstract: Since the Annual Review of Fluid Mechanics first published a review on microorganism locomotion by Jahn & Votta (1972) considerable progress has been made in the understanding of both the biological and the fluid-mechanical processes involved not only in microorganism locomotion but also in other fluid systems utilizing cilia. Much of this knowledge and research, which has been built on the solid foundation of the pioneering work of Sir James Gray (1928, 1968) and Sir Geoffrey Taylor (1951, 1952a,b), has been reported extensively elsewhere, particularly by Gray (1928, 1968), Sleigh (1962), Lighthill (1975), and Wu, Brokaw & Brennen (1975). The subject is now sufficiently broad that it precludes any exhaustive treatment in these few pages. Rather, we restrict this review primarily to a summary of present understanding of the low-Reynolds-number flows associated with microorganism propulsion and the hydromechanics of ciliary systems. In this introductory section we wish to put such fluid-mechanical studies in biological perspective. Section 2 outlines the present status of low-Reynolds-number slender-body theory, and we discuss the application of this theory to biological systems in the final sections.

908 citations

Journal ArticleDOI
08 May 2003-Nature
TL;DR: An experiment in which gentle (linear) bubble oscillations are sufficient to achieve rupture of lipid membranes and the bubble dynamics and the ensuing sonoporation can be accurately controlled.
Abstract: The ability of collapsing (cavitating) bubbles to focus and concentrate energy, forces and stresses is at the root of phenomena such as cavitation damage, sonochemistry or sonoluminescence1, 2. In a biomedical context, ultrasound-driven microbubbles have been used to enhance contrast in ultrasonic images3. The observation of bubble-enhanced sonoporation4, 5, 6?acoustically induced rupture of membranes?has also opened up intriguing possibilities for the therapeutic application of sonoporation as an alternative to cell-wall permeation techniques such as electroporation7 and particle guns8. However, these pioneering experiments have not been able to pinpoint the mechanism by which the violently collapsing bubble opens pores or larger holes in membranes. Here we present an experiment in which gentle (linear) bubble oscillations are sufficient to achieve rupture of lipid membranes. In this regime, the bubble dynamics and the ensuing sonoporation can be accurately controlled. The use of microbubbles as focusing agents makes acoustics on the micrometre scale (microacoustics) a viable tool, with possible applications in cell manipulation and cell-wall permeation as well as in microfluidic devices.

744 citations

Journal ArticleDOI
TL;DR: Direct measurements of the bacterial flow field generated by individual swimming Escherichia coli both far from and near to a solid surface are reported, implying that physical interactions between bacteria are determined by steric collisions and near-field lubrication forces.
Abstract: Bacterial processes ranging from gene expression to motility and biofilm formation are constantly challenged by internal and external noise. While the importance of stochastic fluctuations has been appreciated for chemotaxis, it is currently believed that deterministic long-range fluid dynamical effects govern cell–cell and cell–surface scattering—the elementary events that lead to swarming and collective swimming in active suspensions and to the formation of biofilms. Here, we report direct measurements of the bacterial flow field generated by individual swimming Escherichia coli both far from and near to a solid surface. These experiments allowed us to examine the relative importance of fluid dynamics and rotational diffusion for bacteria. For cell–cell interactions it is shown that thermal and intrinsic stochasticity drown the effects of long-range fluid dynamics, implying that physical interactions between bacteria are determined by steric collisions and near-field lubrication forces. This dominance of short-range forces closely links collective motion in bacterial suspensions to self-organization in driven granular systems, assemblages of biofilaments, and animal flocks. For the scattering of bacteria with surfaces, long-range fluid dynamical interactions are also shown to be negligible before collisions; however, once the bacterium swims along the surface within a few microns after an aligning collision, hydrodynamic effects can contribute to the experimentally observed, long residence times. Because these results are based on purely mechanical properties, they apply to a wide range of microorganisms.

661 citations

Journal ArticleDOI
TL;DR: In this article, the Stokeslet is associated with a singular point force embedded in a Stokes flow and other fundamental singularities can be obtained, including rotlets, stresslets, potential doublets and higher-order poles derived from them.
Abstract: The present study furthcr explores the fundamental singular solutions for Stokes flow that can be useful for constructing solutions over a wide range of free-stream profiles and body shapes. The primary singularity is the Stokeslet, which is associated with a singular point force embedded in a Stokes flow. From its derivatives other fundamental singularities can be obtained, including rotlets, stresslets, potential doublets and higher-order poles derived from them. For treating interior Stokes-flow problems new fundamental solutions are introduced; they include the Stokeson and its derivatives, called the roton and stresson. These fundamental singularities are employed here to construct exact solutions to a number of exterior and interior Stokes-flow problems for several specific body shapes translating and rotating in a viscous fluid which may itself be providing a primary flow. The different primary flows considered here include the uniform stream, shear flows, parabolic profiles and extensional flows (hyperbolic profiles), while the body shapcs cover prolate spheroids, spheres and circular cylinders. The salient features of these exact solutions (all obtained in closed form) regarding the types of singularities required for the construction of a solution in each specific case, their distribution densities and the range of validity of the solution, which may depend on the characteristic Reynolds numbers and governing geometrical parameters, are discussed.

484 citations