arXiv subjects

Michael Dennin

Publications and source records attributed to Michael Dennin.

28 records · Page 2Linked to original sources

arXiv2003

Dependence of Striped Domain Growth on Background Wavenumber

We report on the growth of domains of standing waves in electroconvection in a nematic liquid crystal. An ac voltage is applied to the system, forming an initial state that consists of travelling striped patterns with two different wavenumbers, 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 travelling frequency of the pattern. By varying the modulation frequency, we are able to vary the steady-state, average wavenumber. We characterize the growth by measuring the structure factor for the pattern, the amplitude of the pattern, and the domain-wall length. An interesting feature of the growth is that the evolution of the domain-wall length is strongly affected by changing the selected wavenumber; whereas, the evolution of the structure factor is less sensitive to such changes.

cond-mat.soft↗

arXiv2003

Nonlinear Stress Fluctuation Dynamics of Sheared Disordered Wet Foam

Sheared wet foam, which stores elastic energy in bubble deformations, relaxes stress through bubble rearrangements. The intermittency of bubble rearrangements in 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 $0.07 {\rm s^{-1}}$.

cond-mat.soft↗

arXiv2002

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.

cond-mat.soft↗

arXiv2002

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 of 2-hydroxy-tetracosanoic acid (2-OH TCA), provides a connection between the in-plane rheology of LM's and reversible folding.

cond-mat.soft↗

arXiv2002

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.

nlin.PS↗

arXiv2001

Domain Coarsening in Electroconvection

We report on experimental measurements of the growth of regular domains evolving from an irregular pattern in electroconvection. The late-time growth of the domains is consistent with the size of the domains scaling as $t^n$. We use two isotropic measurements of the domain size: the structure factor and the domain wall length. Measurements using the structure factor are consistent with $t^{1/5}$ growth. Measurements using the domain wall length are consistent with $t^{1/4}$ growth. One source of this discrepancy is the fact that the distribution of local wavenumbers is approximately independent of the domain size. In addition, we measure the anisotropy of the growing domains.

cond-mat.soft↗

arXiv2000

Temporal Modulation of the Control Parameter in Electroconvection in the Nematic Liquid Crystal I52

I report on the effects of a periodic modulation of the control parameter on electroconvection in the nematic liquid crystal I52. Without modulation, the primary bifurcation from the uniform state is a direct transition to a state of spatiotemporal chaos. This state is the result of the interaction of four, degenerate traveling modes: right and left zig and zag rolls. Periodic modulations of the driving voltage at approximately twice the traveling frequency are used. For a large enough modulation amplitude, standing waves that consist of only zig or zag rolls are stabilized. The standing waves exhibit regular behavior in space and time. Therefore, modulation of the control parameter represents a method of eliminating spatiotemporal chaos. As the modulation frequency is varied away from twice the traveling frequency, standing waves that are a superposition of zig and zag rolls, i.e. standing rectangles, are observed. These results are compared with existing predictions based on coupled complex Ginzburg-Landau equations.

nlin.PS↗

arXiv2000

Direct observation of twist mode in electroconvection in I52

I report on the direct observation of a uniform twist mode of the director field in electroconvection in I52. Recent theoretical work suggests that such a uniform twist mode of the director field is responsible for a number of secondary bifurcations in both electroconvection and thermal convection in nematics. I show here evidence that the proposed mechanisms are consistent with being the source of the previously reported SO2 state of electroconvection in I52. The same mechanisms also contribute to a tertiary Hopf bifurcation that I observe in electroconvection in I52. There are quantitative differences between the experiment and calculations that only include the twist mode. These differences suggest that a complete description must include effects described by the weak-electrolyte model of electroconvection.

cond-mat.soft↗

arXiv1999

Reversible Random Sequential Adsorption of Dimers on a Triangular Lattice

We report on simulations of reversible random sequential adsorption of dimers on three different lattices: a one-dimensional lattice, a two-dimensional triangular lattice, and a two-dimensional triangular lattice with the nearest neighbors excluded. In addition to the adsorption of particles at a rate K+, we allow particles to leave the surface at a rate K-. The results from the one-dimensional lattice model agree with previous results for the continuous parking lot model. In particular, the long-time behavior is dominated by collective events involving two particles. We were able to directly confirm the importance of two-particle events in the simple two-dimensional triangular lattice. For the two-dimensional triangular lattice with the nearest neighbors excluded, the observed dynamics are consistent with this picture. The two-dimensional simulations were motivated by measurements of Ca++ binding to Langmuir monolayers. The two cases were chosen to model the effects of changing pH in the experimental system.

cond-mat.stat-mech↗

arXiv1999

Effect of Subphase Ca++ Ions on the Viscoelastic Properties of Langmuir Monolayers

It is known that the presence of cations like Ca++ or Pb++ in the water subphase alters the pressure-area isotherms for fatty acid monolayers. The corresponding lattice constant changes have been studied using x-ray diffraction. Reflection-absorption spectroscopy has been used to probe the chemical composition of the film. We report on the first measurements of the time evolution of the shear viscosity of arachidic acid monolayers in the presence of Ca++ ions in the subphase. We find that the introduction of Ca++ ions to the water subphase results in an increase of the film's viscosity by at least three orders of magnitude. This increase occurs in three distinct stages. First, there is a rapid change in the viscosity of up to one order of magnitude. This is followed by two periods, with very different time constants, of a relatively slow increase in the viscosity over the next 10 or more hours. The corresponding time constants for this rise decrease as either the subphase pH or Ca++ concentration is increased.

physics.chem-ph↗