Showing papers by "Julia M. Yeomans published in 2010"
TL;DR: Numerical modelling of colloidal particles in chiral nematics with cubic symmetry (blue phases) within the framework of the Landau-de Gennes free energy shows the interaction potential is shown to be highly anisotropic and have threefold rotational symmetry.
Abstract: We present numerical modelling of colloidal particles in chiral nematics with cubic symmetry (blue phases) within the framework of the Landau-de Gennes free energy. The interaction potential of a single, nano-sized colloidal particle with a −1/2 disclination line is calculated as a generic trapping mechanism for particles within the cholesteric blue phases. The interaction potential is shown to be highly anisotropic and have threefold rotational symmetry. We discuss the equilibration of the colloidal texture with respect to particle positions and the unit cell size of the blue phase. We also describe how preservation of the liquid crystal volume and the number of particles allows blue phase colloidal structures with different unit cell sizes and configurations to be compared numerically.
94 citations
TL;DR: It is argued that a tank treading advance from post to post replaces the diffusive relaxation that allows the contact line to move on smooth surfaces, and hence drops move on superhydrophobic surfaces more quickly than on smooth geometries.
Abstract: We investigate the dynamics of micron-scale drops pushed across a hydrophobic or superhydrophobic surface. The velocity profile across the drop varies from quadratic to linear with increasing height, indicating a crossover from a sliding to a rolling motion. We identify a mesoscopic slip capillary number which depends only on the motion of the contact line and the shape of the drop, and show that the angular velocity of the rolling increases with increasing viscosity. For drops on superhydrophobic surfaces we argue that a tank treading advance from post to post replaces the diffusive relaxation that allows the contact line to move on smooth surfaces. Hence drops move on superhydrophobic surfaces more quickly than on smooth geometries.
63 citations
TL;DR: A systematic theoretical description of anomalous tracer diffusion in active suspensions is developed, based on a simplified tracer-swimmer interaction model that captures the typical distance scaling of a microswimmer's flow field and shows that the experimentally observed non-Gaussian tails are generic and arise owing to a combination of truncated Lévy statistics for the velocity field and algebraically decaying time correlations in the fluid.
Abstract: Swimming microorganisms rely on effective mixing strategies to achieve efficient nutrient influx. Recent experiments, probing the mixing capability of unicellular biflagellates, revealed that passive tracer particles exhibit anomalous non-Gaussian diffusion when immersed in a dilute suspension of self-motile Chlamydomonas reinhardtii algae. Qualitatively, this observation can be explained by the fact that the algae induce a fluid flow that may occasionally accelerate the colloidal tracers to relatively large velocities. A satisfactory quantitative theory of enhanced mixing in dilute active suspensions, however, is lacking at present. In particular, it is unclear how non-Gaussian signatures in the tracers' position distribution are linked to the self-propulsion mechanism of a microorganism. Here, we develop a systematic theoretical description of anomalous tracer diffusion in active suspensions, based on a simplified tracer-swimmer interaction model that captures the typical distance scaling of a microswimmer's flow field. We show that the experimentally observed non-Gaussian tails are generic and arise due to a combination of truncated L\'evy statistics for the velocity field and algebraically decaying time correlations in the fluid. Our analytical considerations are illustrated through extensive simulations, implemented on graphics processing units to achieve the large sample sizes required for analyzing the tails of the tracer distributions.
57 citations
TL;DR: In this paper, the scattering of a tracer particle in different types of time-dependent, hydrodynamic flow fields was analyzed and compared with self-motile, multi-sphere swimmers, showing that force-free swimmers generate loop-shaped tracer trajectories.
Abstract: Understanding the stochastic dynamics of tracer particles in active fluids is important for identifying the physical properties of flow generating objects such as colloids, bacteria or algae. Here, we study both analytically and numerically the scattering of a tracer particle in different types of time-dependent, hydrodynamic flow fields. Specifically, we compare the tracer motion induced by an externally driven colloid with the one generated by various self-motile, multi-sphere swimmers. Our results suggest that force-free swimmers generically induce loop-shaped tracer trajectories. The specific topological structure of these loops is determined by the hydrodynamic properties of the microswimmer. Quantitative estimates for typical experimental conditions imply that the loops survive on average even if Brownian motion effects are taken into account.
56 citations
TL;DR: It is found, in agreement with ref 17 , that the local post concentration is a primary factor in controlling the depinning angle and the numerical results agree well with recent experiments.
Abstract: We use mesoscale simulations to study the depinning of a receding contact line on a superhydrophobic surface patterned by a regular array of posts. For the simulations to be feasible, we introduce a novel geometry where a column of liquid dewets a capillary bounded by a superhydrophobic plane that faces a smooth hydrophilic wall of variable contact angle. We present results for the dependence of the depinning angle on the shape and spacing of the posts and discuss the form of the meniscus at depinning. We find, in agreement with ref 17, that the local post concentration is a primary factor in controlling the depinning angle and show that the numerical results agree well with recent experiments. We also present two examples of metastable pinned configurations where the posts are partially wet.
53 citations
TL;DR: In this paper, the scattering of a tracer particle in different types of time-dependent, hydrodynamic flow fields was analyzed and compared with self-motile, multi-sphere swimmers, showing that force-free swimmers generate loop-shaped tracer trajectories.
Abstract: Understanding the stochastic dynamics of tracer particles in active fluids is important for identifying the physical properties of flow generating objects such as colloids, bacteria or algae. Here, we study both analytically and numerically the scattering of a tracer particle in different types of time-dependent, hydrodynamic flow fields. Specifically, we compare the tracer motion induced by an externally driven colloid with the one generated by various self-motile, multi-sphere swimmers. Our results suggest that force-free swimmers generically induce loop-shaped tracer trajectories. The specific topological structure of these loops is determined by the hydrodynamic properties of the microswimmer. Quantitative estimates for typical experimental conditions imply that the loops survive on average even if Brownian motion effects are taken into account.
44 citations
TL;DR: In this paper, the authors compare the dynamical behavior of torus and even-twist knots in polymers under tension, and find that the torus knots diffuse faster than the even twist knots.
Abstract: We use computer simulations to compare the dynamical behaviour of torus and even-twist knots in polymers under tension. The knots diffuse through a mechanism similar to reptation. Their friction coefficients grow linearly with average knot length for both knot types. For similar complexity, however, the torus knots diffuse faster than the even-twist knots. The knot length autocorrelation function exhibits a relaxation time that can be linked to a breathing mode. Its timescale depends on knot type, being typically longer for torus than for even-twist knots. These differences in dynamical behaviour are interpreted in terms of topological features of the knots.
42 citations
TL;DR: It is found that now both singlets and doublets can exhibit superhydrophobic behavior on hydrophilic hairs, and the results can be modified to describe an array of hairs in three dimensions.
Abstract: We investigate the wetting properties of surfaces patterned with fine elastic hairs, with an emphasis on identifying superhydrophobic states on hydrophilic hairs. We formulate a 2D model of a large drop in contact with a row of equispaced elastic hairs and, by minimizing the free energy of the model, identify the stable and metastable states. In particular, we concentrate on partially suspended states, where the hairs bend to support the drop--singlet states, where all hairs bend in the same direction, and doublet states, where neighboring hairs bend in opposite directions--and find the limits of stability of these configurations in terms of the material contact angle, hair flexibility, and system geometry. The drop can remain suspended in a singlet state at hydrophilic contact angles, but doublets exist only when the hairs are hydrophobic. The system is more likely to evolve into a singlet state if the hairs are inclined at the root. We discuss how, under limited circumstances, the results can be modified to describe an array of hairs in three dimensions. We find that now both singlets and doublets can exhibit superhydrophobic behavior on hydrophilic hairs. We discuss the limitations of our approach and the directions for future work.
36 citations
TL;DR: In this article, the authors describe and analyze CUDA simulations of hydrodynamic interactions in active dumbbell suspensions and show that the stroke-averaged model yields a relatively accurate description down to distances of only a few times the dumbbell's length.
21 citations
01 Jan 2010
TL;DR: The goal is to engineer “lab on a chip” devices, where numerous biological and chemical experiments can be performed rapidly, and in parallel, while consuming little reagent.
Abstract: Recently there has been a huge effort in the scientific community to miniaturise fluidic operations to micron and nanoscales [1]. This has changed the way scientists think about fluids, and it potentially has far-reaching technological implications, analogous to the miniaturization of electronics. The goal is to engineer “lab on a chip” devices, where numerous biological and chemical experiments can be performed rapidly, and in parallel, while consuming little reagent.
19 citations
TL;DR: In this paper, coarse-grained simulations are used to demonstrate that knotted filaments in shear flow at zero Reynolds number exhibit remarkably rich dynamic behaviour, and the interplay between shear forces and the flexibility of the filament leads to intricate regular and chaotic modes of motion that can be divided into distinct families.
Abstract: Coarse-grained simulations are used to demonstrate that knotted filaments in shear flow at zero Reynolds number exhibit remarkably rich dynamic behaviour. For stiff filaments that are weakly deformed by the shear forces, the knotted filaments rotate like rigid objects in the flow. But away from this regime the interplay between shear forces and the flexibility of the filament leads to intricate regular and chaotic modes of motion that can be divided into distinct families. The set of accessible mode families depends to first order on a dimensionless number that relates the filament length, the elastic modulus, the friction per unit length and the shear rate.
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TL;DR: In this paper, the dynamics of micron-scale drops pushed across a hydrophobic or superhydrophobic surface were investigated and the velocity profile across the drop varies from quadratic to linear with increasing height, indicating a crossover from a sliding to a rolling motion.
Abstract: We investigate the dynamics of micron-scale drops pushed across a hydrophobic or superhydrophobic surface. The velocity profile across the drop varies from quadratic to linear with increasing height, indicating a crossover from a sliding to a rolling motion. We identify a mesoscopic slip capillary number which depends only on the motion of the contact line and the shape of the drop, and show that the angular velocity of the rolling increases with increasing viscosity. For drops on superhydrophobic surfaces we argue that a tank treading advance from post to post replaces the diffusive relaxation that allows the contact line to move on smooth surfaces. Hence drops move on superhydrophobic surfaces more quickly than on smooth geometries.
TL;DR: In this paper, the authors studied domain growth in nematic liquid crystals using a lattice Boltzmann algorithm to solve the full three-dimensional equations of hydrodynamics.
Abstract: We have studied domain growth in nematic liquid crystals using a lattice Boltzmann algorithm to solve the full, three-dimensional equations of hydrodynamics. An initially cylindrical V (bend) domain in an H (splay) state grows or shrinks anisotropically in agreement with experiment. A disclination loop forms at the mid-point of the wall surrounding the domain. We argue that different director configurations at different points on the loop lead to velocity anisotropy and show that both elastic effects and backflow are relevant. We discuss the dependence of the domain wall velocity on surface tilt and on the magnitude of an applied electric field.
TL;DR: A review of hydrodynamic interactions of low Reynolds number microswimmers is presented in this article, with particular attention paid to those aspects that are generic, applicable to all micro swimmers and not only to the simple models considered.
Abstract: We consider the hydrodynamic interactions of low Reynolds number microswimmers, presenting a review of recent work based upon models of linked sphere swimmers Particular attention is paid to those aspects that are generic, applicable to all microswimmers and not only to the simple models considered The importance of the relative phase in swimmer–swimmer interactions is emphasised, as is the role of simple symmetry arguments in both understanding and constraining the hydrodynamic properties of microswimmers
TL;DR: In this article, a mesoscale simulation is used to demonstrate the feasibility of a novel microfluidic valve, which exploits Gibbs' pinning in microchannels patterned by posts or ridges, together with electrowetting.
Abstract: We use mesoscale simulations to demonstrate the feasibility of a novel microfluidic valve, which exploits Gibbs' pinning in microchannels patterned by posts or ridges, together with electrowetting.
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TL;DR: In this article, a mesoscale simulation is used to demonstrate the feasibility of a novel microfluidic valve, which exploits Gibbs' pinning in microchannels patterned by posts or ridges, together with electrowetting.
Abstract: We use mesoscale simulations to demonstrate the feasibility of a novel microfluidic valve, which exploits Gibbs' pinning in microchannels patterned by posts or ridges, together with electrowetting.
TL;DR: In this paper, the authors examined the possibilities to use the intrinsic 3D defect networks in blue phases I and II as arrays of trapping sites for colloidal particles, and they first described the trapping mechanism on the case of a single discilination line.
Abstract: We examine the possibilities to use the intrinsic 3D defect networks in blue phases I and II as arrays of trapping sites for
colloidal particles. Our approach based on the phenomenological Landau-de Gennes description and topological theory
has proven to be extremely useful in dealing with nematic colloids. A perturbed orientational order leads to effective
anisotropic long range inter-particle coupling and consequently to numerous organizations of colloidal particles not
present in simple liquids. Recent developments that led to the blue phases with extended stability range make them more
attractive for use. In these phases the competition between nematic ordering and intrinsic tendency to form double
twisted deformations yields complex director patterns and disclination networks. The spatially deformed order that
mediates the attraction of particles to the network sets the ground for a possible self-assembling of 3D superstructures
with extended stability ranges. Here we first describe the trapping mechanism on the case of a single discilination line
and then use the results to demonstrate the trapping in the blue phase II. Effects of particle sizes ranging from submicron
to 50 nanometers are examined. The assembling in blue phases is expected to form photonic crystals that can be easily
manipulated via affecting the liquid crystal matrix and/or colloidal particles.
TL;DR: In this paper, the effect of knot type on the properties of a ring polymer confined to a slit was investigated, and it was shown that the more complex the knot, the more the force exerted by the polymer on the walls is decreased compared to an unknotted polymer of the same length.
Abstract: We investigate the effect of knot type on the properties of a ring polymer confined to a slit. For relatively wide slits, the more complex the knot, the more the force exerted by the polymer on the walls is decreased compared to an unknotted polymer of the same length. For more narrow slits the opposite is true. The crossover between these two regimes is, to first order, at smaller slit width for more complex knots. However, knot topology can affect these trends in subtle ways. Besides the force exerted by the polymers, we also study other quantities such as the monomer-density distribution across the slit and the anisotropic radius of gyration.
TL;DR: In this paper, the authors investigate the wetting properties of surfaces patterned with fine elastic hairs, with an emphasis on identifying superhydrophobic states on hydrophilic hairs, and find the limits of stability of these configurations in terms of material contact angle, hair flexibility, and system geometry.
Abstract: We investigate the wetting properties of surfaces patterned with fine elastic hairs, with an emphasis on identifying superhydrophobic states on hydrophilic hairs. We formulate a two dimensional model of a large drop in contact with a row of equispaced elastic hairs and, by minimising the free energy of the model, identify the stable and metastable states. In particular we concentrate on "partially suspended" states, where the hairs bend to support the drop -- singlet states where all hairs bend in the same direction, and doublet states where neighbouring hairs bend in opposite directions -- and find the limits of stability of these configurations in terms of material contact angle, hair flexibility, and system geometry. The drop can remain suspended in a singlet state at hydrophilic contact angles, but doublets exist only when the hairs are hydrophobic. The system is more likely to evolve into a singlet state if the hairs are inclined at the root. We discuss how, under limited circumstances, the results can be modified to describe an array of hairs in three dimensions. We find that now both singlets and doublets can exhibit superhydrophobic behaviour on hydrophilic hairs. We discuss the limitations of our approach and the directions for future work.