Biomedical subjects

Michael Dennin

Publications and source records attributed to Michael Dennin.

8 recordsLinked to original sources

PubMed2004

Statistics of bubble rearrangements in a slowly sheared two-dimensional foam.

Many physical systems exhibit plastic flow when subjected to slow steady shear. A unified picture of plastic flow is still lacking; however, there is an emerging theoretical understanding of such flows based on irreversible motions of the constituent "particles" of the material. Depending on the specific system, various irreversible events have been studied, such as T1 events in foam and shear transformation zones (STZ's) in amorphous solids. This paper presents an experimental study of the T1 events in a model, two-dimensional foam: bubble rafts. In particular, I report on the connection between the distribution of T1 events and the behavior of the average stress and average velocity profiles during both the initial elastic response of the bubble raft and the subsequent plastic flow at sufficiently high strains.

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PubMed2004

Dislocation dynamics in an anisotropic stripe pattern.

The dynamics of dislocations confined to grain boundaries in a striped system are studied using electroconvection in the nematic liquid crystal N4. In electroconvection, a striped pattern of convection rolls forms for sufficiently high driving voltages. We consider the case of a rapid change in the voltage that takes the system from a uniform state to a state consisting of striped domains with two different wave vectors. The domains are separated by domain walls along one axis and a grain boundary of dislocations in the perpendicular direction. The pattern evolves through dislocation motion parallel to the domain walls. We report on features of the dislocation dynamics. The kinetics of the domain motion is quantified using three measures: dislocation density, average domain wall length, and total domain wall length per area. All three quantities exhibit behavior consistent with power-law evolution in time, with the defect density decaying as t(-1/3), the average domain wall length growing as t(1/3), and the total domain wall length decaying as t(-1/5). The two different exponents are indicative of the anisotropic growth of domains in the system.

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PubMed2004

Dependence of domain wall dynamics on background wave number.

We report on the growth of domains of standing waves in electroconvection in a nematic liquid crystal, focusing on the evolution of domain walls. An ac voltage is applied to the system, forming an initial state that consists of traveling striped patterns with two different orientations, zig and zag rolls. The standing waves are generated by suddenly applying a periodic modulation of the amplitude of the applied voltage that is approximately resonant with the traveling frequency of the pattern. By varying the modulation frequency, we are able to vary the steady-state, average wave number. We characterize the evolution of the domain walls as a function of the average background wave number by measuring the total area and length of domain walls present in the system as a function of time. We find that as the background wave number is varied away from the "natural" wave number for the pattern, the evolution of the domain walls occurs at a faster rate.

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PubMed2003

Nonlinear stress and fluctuation dynamics of sheared disordered wet foam.

A sheared wet foam, which stores elastic energy in bubble deformations, relaxes stress through bubble rearrangements. The intermittency of bubble rearrangements in the foam leads to effectively stochastic drops in stress that are followed by periods of elastic increase. We investigate global characteristics of highly disordered foams over three decades of strain rate and almost two decades of system size. We characterize the behavior using a range of measures: average stress, distribution of stress drops, rate of stress drops, and a normalized fluctuation intensity. There is essentially no dependence on system size. As a function of strain rate, there is a change in behavior around shear rates of 0.07 s(-1).

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PubMed2003

Patterns of electroconvection in the nematic liquid crystal N4.

Electroconvection using the liquid crystal N4 is studied as a function of two control parameters: the applied frequency and the applied voltage. As a function of voltage, there is a rich series of bifurcations that takes the system from stationary rolls to chaos. As a function of the frequency, the initial pattern changes from stationary oblique rolls at low frequencies to stationary normal rolls at higher frequencies. There is also a change in the secondary bifurcations. In particular, we observe that the bimodal-varicose instability is replaced by the skewed-varicose instability as the applied frequency is increased. Comparisons with theoretical predictions are made.

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PubMed2002

Folding Langmuir monolayers.

The maximum pressure a two-dimensional surfactant monolayer is able to withstand is limited by the collapse instability towards formation of three-dimensional material. We propose a new description for reversible collapse based on a mathematical analogy between the formation of folds in surfactant monolayers and the formation of Griffith Cracks in solid plates under stress. The description, which is tested in a combined microscopy and rheology study of the collapse of a single-phase Langmuir monolayer (LM) of 2-hydroxy-tetracosanoic acid (2-OH TCA), provides a connection between the in-plane rheology of LMs and reversible folding.

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PubMed2002

Shear-induced stress relaxation in a two-dimensional wet foam.

We report on experimental measurements of the flow behavior of a wet, two-dimensional foam under conditions of slow, steady shear. The initial response of the foam is elastic. Above the yield strain, the foam begins to flow. The flow consists of irregular intervals of elastic stretch followed by sudden reductions of the stress, i.e., stress drops. We report on the distribution of the stress drops as a function of the applied shear rate. We also comment on our results in the context of various two-dimensional models of foams.

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PubMed2002

Modulation of localized states in electroconvection.

We report on the effects of temporal modulation of the driving force on a particular class of localized states, known as worms, that have been observed in electroconvection in nematic liquid crystals. The worms consist of the superposition of traveling waves and have been observed to have unique, small widths, but to vary in length. The transition from the pure conduction state to worms occurs via a backward bifurcation. A possible explanation of the formation of the worms has been given in terms of coupled amplitude equations. Because the worms consist of the superposition of traveling waves, temporal modulation of the control parameter is a useful probe of the dynamics of the system. We observe that temporal modulation increases the average length of the worms and stabilizes worms below the transition point in the absence of modulation.

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