Showing papers by "Julia M. Yeomans published in 2012"
TL;DR: In this paper, the authors combine experiments, particle simulations, and continuum theory to identify the statistical properties of self-sustained meso-scale turbulence in active systems, and propose a minimal continuum model for incompressible bacterial flow.
Abstract: Turbulence is ubiquitous, from oceanic currents to small-scale biological and quantum systems. Self-sustained turbulent motion in microbial suspensions presents an intriguing example of collective dynamical behavior among the simplest forms of life and is important for fluid mixing and molecular transport on the microscale. The mathematical characterization of turbulence phenomena in active nonequilibrium fluids proves even more difficult than for conventional liquids or gases. It is not known which features of turbulent phases in living matter are universal or system-specific or which generalizations of the Navier–Stokes equations are able to describe them adequately. Here, we combine experiments, particle simulations, and continuum theory to identify the statistical properties of self-sustained meso-scale turbulence in active systems. To study how dimensionality and boundary conditions affect collective bacterial dynamics, we measured energy spectra and structure functions in dense Bacillus subtilis suspensions in quasi-2D and 3D geometries. Our experimental results for the bacterial flow statistics agree well with predictions from a minimal model for self-propelled rods, suggesting that at high concentrations the collective motion of the bacteria is dominated by short-range interactions. To provide a basis for future theoretical studies, we propose a minimal continuum model for incompressible bacterial flow. A detailed numerical analysis of the 2D case shows that this theory can reproduce many of the experimentally observed features of self-sustained active turbulence.
803 citations
TL;DR: In this paper, the authors calculate the Darwin drift, the total volume swept out by a material sheet of tracers, initially perpendicular to the swimming path, during the swimmer motion.
Abstract: We discuss the path of a tracer particle as a microswimmer moves past on an infinite straight trajectory. If the tracer is sufficiently far from the path of the swimmer it moves in a closed loop. As the initial distance between the tracer and the path of the swimmer $\rho$ decreases, the tracer is displaced a small distance backwards (relative to the direction of the swimmer velocity). For much smaller tracer-swimmer separations, however, the tracer displacement becomes positive and diverges as $\rho \to 0$. To quantify this behaviour we calculate the Darwin drift, the total volume swept out by a material sheet of tracers, initially perpendicular to the swimmer path, during the swimmer motion. We find that the drift can be written as the sum of a {\em universal} term which depends on the quadrupolar flow field of the swimmer, together with a non-universal contribution given by the sum of the volumes of the swimmer and its wake. The formula is compared to exact results for the squirmer model and to numerical calculations for a more realistic model swimmer.
55 citations
TL;DR: In this article, the authors proposed a simple model for a swimmer that undergoes circular motion, generalising the model of a linear swimmer proposed by Najafi and Golestanian.
Abstract: Swimming in circles occurs in a variety of situations at low Reynolds number. Here we propose a simple model for a swimmer that undergoes circular motion, generalising the model of a linear swimmer proposed by Najafi and Golestanian (Phys. Rev. E 69, 062901 (2004)). Our model consists of three solid spheres arranged in a triangular configuration, joined by two links of time-dependent length. For small strokes, we discuss the motion of the swimmer as a function of the separation angle between its links. We find that swimmers describe either clockwise or anticlockwise circular motion depending on the tilting angle in a non-trivial manner. The symmetry of the swimmer leads to a quadrupolar decay of the far flow field. We discuss the potential extensions and experimental realisation of our model.
51 citations
TL;DR: The nematic phase of rodlike f d-virus particles confined to channels with wedge-structured walls is studied and a simple method to estimate the splay-to-bend elasticity ratios of the virus is provided.
Abstract: We study the nematic phase of rodlike f d-virus particles confined to channels with wedge-structured walls. Using laser scanning confocal microscopy we observe a splay-to-bend transition at the single particle level as a function of the wedge opening angle. Lattice Boltzmann simulations reveal the underlying origin of the transition and its dependence on nematic elasticity and wedge geometry. Our combined work provides a simple method to estimate the splay-to-bend elasticity ratios of the virus and offers a way to control the position of defects through the confining boundary conditions.
48 citations
TL;DR: In this paper, the de Gennes blob model and mesoscopic numerical simulations were used to estimate the threshold flux for the translocation of chains of different number of monomers through nanochannels.
Abstract: We consider the flow-driven translocation of single polymer chains through nanochannels. Using analytical calculations based on the de Gennes blob model and mesoscopic numerical simulations, we estimate the threshold flux for the translocation of chains of different number of monomers. The translocation of the chains is controlled by the competition between entropic and hydrodynamic effects, which set a critical penetration length for the chain before it can translocate through the channel. We demonstrate that the polymers show two different translocation regimes depending on how their length under confinement compares to the critical penetration length. For polymer chains longer than the threshold, the translocation process is insensitive to the number of monomers in the chain as predicted in Sakaue et al., Euro. Phys. Lett., 2005, 72, 83. However, for chains shorter than the critical length we show that the translocation process is strongly dependent on the length of the chain. We discuss the possible relevance of our results to biological transport.
44 citations
TL;DR: The defects' speed is interpreted in terms of their overlap and the speed asymmetry as arising from backflow effects and anisotropy in the elastic constants, which is found to be essentially constant for varying external conditions which do not change the material properties of the liquid crystal material.
Abstract: Umbilic defects of strength s=±1 were induced in a nematic liquid crystal with negative dielectric anisotropy, confined to Hele-Shaw cells with homeotropic boundary conditions, and their annihilation dynamics followed experimentally. The speeds of individual defects of annihilating defect pairs with strengths of equal magnitude and opposite sign were determined as a function of several externally applied parameters, such as cell gap, electric field amplitude, frequency, and temperature. It was shown that annihilating defects do not approach each other at equal speeds, but that a speed anisotropy is observed, with the positive defect moving faster than the negative one. The defects move more slowly as the strength of the applied electric field or the cell gap is increased. The speed anisotropy is found to be essentially constant for varying external conditions which do not change the material properties of the liquid crystal material, i.e., confinement, electric field amplitude, or frequency. Only for applied conditions that change material properties, such as temperature changing viscosity, does the speed anisotropy vary. The annihilation dynamics was also simulated numerically giving good qualitative agreement with the experiments. Using insight gained from the simulations we interpret the defects' speed in terms of their overlap and the speed asymmetry as arising from backflow effects and anisotropy in the elastic constants.
44 citations
Posted Content•
TL;DR: A simple model for a swimmer that undergoes circular motion is proposed, generalising the model of a linear swimmer proposed by Najafi and Golestanian and finding that swimmers describe either clockwise or anticlockwise circular motion depending on the tilting angle in a non-trivial manner.
Abstract: Swimming in circles occurs in a variety of situations at low Reynolds number. Here we propose a simple model for a swimmer that undergoes circular motion, generalising the model of a linear swimmer proposed by Najafi and Golestanian (Phys. Rev. E 69, 062901 (2004)). Our model consists of three solid spheres arranged in a triangular configuration, joined by two links of time-dependent length. For small strokes, we discuss the motion of the swimmer as a function of the separation angle between its links. We find that swimmers describe either clockwise or anticlockwise circular motion depending on the tilting angle in a non-trivial manner. The symmetry of the swimmer leads to a quadrupolar decay of the far flow field. We discuss the potential extensions and experimental realisation of our model.
34 citations
Posted Content•
TL;DR: In this article, the authors show how water drops, produced by ink-jet printing, spread on surfaces patterned with lattices of diamond or triangular posts and observe drop shapes with 3,4 and 6-fold symmetry, depending on both the symmetry of the lattice and the shape of the posts.
Abstract: We present results showing how water drops, produced by ink-jet printing, spread on surfaces patterned with lattices of diamond or triangular posts. Considering post widths typically ~7 m and lattice spacings between 15-40 m, we observe drop shapes with 3,4 and 6-fold symmetry, depending on both the symmetry of the lattice and the shape of the posts. This is a result of the different mechanisms of interface pinning and depinning which depend on the direction of the contact line motion with respect to the post shape. Lattice Boltzmann simulations are used to describe these mechanisms in detail for triangular posts. We also follow the motion of the contact line as the drops evaporate showing that they tend to return to their original shape. To explain this we show that the easy direction for movement is the same for spreading and drying drops. We compare the behaviour of small drops with that of larger drops created by jetting several drops at the same position. We find that the contact line motion is unexpectedly insensitive to drop volume, even when a spherical cap of fluid forms above the posts. The findings are relevant to microfluidic applications and to the control of drop shapes in ink-jet printing.
29 citations
TL;DR: In this article, the de Gennes blob model of confined polymers is extended to the case of nanochannels and it is shown that the injection of polymer chains into a nanochannel becomes easier as the channel becomes narrower.
Abstract: We show that the injection of polymer chains into nanochannels becomes easier as the channel becomes narrower. This counter intuitive result arises because of a decrease in the diffusive time scale of the chains with increasing confinement. The results are obtained by extending the de Gennes blob model of confined polymers, and confirmed by hybrid molecular dynamics–lattice-Boltzmann simulations.
17 citations
TL;DR: Numerical results for curved posts with a horizontal section at their ends show that these are more efficient in stabilising the Cassie state than straight posts, and identify whether the interface first depins from the post sides or the post tips.
Abstract: An important feature in the design of superhydrophobic surfaces is their robustness against collapse from the Cassie-Baxter configuration to the Wenzel state Upon such a transition a surface loses its properties of low adhesion and friction We describe how to adapt the Surface Evolver algorithm to predict the parameters and mechanism of the collapse transition on posts of arbitrary shape In particular, contributions to the free energy evaluated over the solid-liquid surface are reduced to line integrals to give good convergence The algorithm is validated for straight, vertical and inclined, posts Numerical results for curved posts with a horizontal section at their ends show that these are more efficient in stabilising the Cassie state than straight posts, and identify whether the interface first depins from the post sides or the post tips
8 citations
TL;DR: In this article, the Surface Evolver algorithm is used to predict the parameters and mechanism of the collapse transition on posts of arbitrary shape, including straight, vertical and inclined, posts.
Abstract: An important feature in the design of superhydrophobic surfaces is their robustness against collapse from the Cassie–Baxter configuration to the Wenzel state. Upon such a transition a surface loses its properties of low adhesion and friction. We describe how to adapt the Surface Evolver algorithm to predict the parameters and mechanism of the collapse transition on posts of arbitrary shape. In particular, contributions to the free energy evaluated over the solid–liquid surface are reduced to line integrals to give good convergence. The algorithm is validated for straight, vertical and inclined, posts. Numerical results for curved posts with a horizontal section at their ends show that these are more efficient in stabilizing the Cassie state than straight posts, and identify whether the interface first depins from the post sides or the post tips.
TL;DR: In this paper, the authors present and interpret lattice Boltzmann simulations of spreading on surfaces patterned with polygonal posts, and show that the mechanism of pinning and depinning differs with the direction of advance, leading to anisotropic spreading within a certain range of material contact angles.
Abstract: We present and interpret lattice Boltzmann simulations of thick films spreading on surfaces patterned with polygonal posts. We show that the mechanism of pinning and depinning differs with the direction of advance, and demonstrate that this leads to anisotropic spreading within a certain range of material contact angles.
Journal Article•
TL;DR: In this paper, the de Gennes blob model of confined polymers is extended to the case of nanochannels and it is shown that the injection of polymer chains into a nanochannel becomes easier as the channel becomes narrower.
Abstract: We show that the injection of polymer chains into nanochannels becomes easier as the channel becomes narrower. This counter intuitive result arises because of a decrease in the diffusive time scale of the chains with increasing confinement. The results are obtained by extending the de Gennes blob model of confined polymers, and confirmed by hybrid molecular dynamics - lattice-Boltzmann simulations.