Polar active matter of self-propelled particles sustain spontaneous flows through the full-intege... more Polar active matter of self-propelled particles sustain spontaneous flows through the full-integer topological defects. We study theoretically the incompressible flow profiles around ±1 defects induced by polar and dipolar active...
We adapt the Halperin–Mazenko formalism to analyze two-dimensional active nematics coupled to a g... more We adapt the Halperin–Mazenko formalism to analyze two-dimensional active nematics coupled to a generic fluid flow. The governing hydrodynamic equations lead to evolution laws for nematic topological defects and their corresponding density fields. We find that ±1/2 defects are propelled by the local fluid flow and by the nematic orientation coupled with the flow shear rate. In the overdamped and compressible limit, we recover the previously obtained active self-propulsion of the +1/2 defects. Non-local hydrodynamic effects are primarily significant for incompressible flows, for which it is not possible to eliminate the fluid velocity in favor of the local defect polarization alone. For the case of two defects with opposite charge, the non-local hydrodynamic interaction is mediated by non-reciprocal pressure-gradient forces. Finally, we derive continuum equations for a defect gas coupled to an arbitrary (compressible or incompressible) fluid flow.
The collective motion of driven or self-propelled interacting units is in many natural systems kn... more The collective motion of driven or self-propelled interacting units is in many natural systems known to produce complex patterns. This thesis considers two continuum field theories commonly used in describing pattern formation and dynamics: The first one, the phase field crystal model, which describes the dynamical and equilibrium properties of crystalline material, is used to study the coarsening dynamics of polycrystalline materials in two and three dimensions. A generalization introducing a faster elastic relaxation time scale is then used to study the plastic deformation and dislocation dynamics of single crystals. Secondly, a continuum theory describing mesoscopic turbulence of biological active matter, which is used to study long-range ordered vorticity patterns generated by cell divisions in a endothelial cell layer.
Abstract We perform an experimental analysis of stratified, co-current gas-liquid flow in a strai... more Abstract We perform an experimental analysis of stratified, co-current gas-liquid flow in a straight pipe, with the aim of investigating secondary flow structures in the gas-phase. Planar PIV (particle image velocimetry) of the pipe centerline as well as stereoscopic PIV of the pipe cross section is performed, considering cases where the interface is either rough (wavy) or flat. Results demonstrate that the interface morphology determines the direction of the secondary flow patterns. Over a flat interface, the secondary flow cells are directed up along the pipe centerline and down along the pipe walls. When the interface is rough (wavy), the direction of the secondary flow cells is reversed. Normalized by the peak axial velocity, the vertical velocity profile in the pipe centerline is seen to collapse to a common curve when the interface is flat, while results indicate that the normalized strength of the secondary flows increase with increasing surface roughness. An assessment of the Reynolds stresses in the cross section is performed, and the direction of the secondary flow cells are found to be well predicted by the dominant gradients of the radial and circumferential Reynolds stresses (τrr and τθθ) in the radial and circumferential direction respectively.
Polar active matter of self-propelled particles sustain spontaneous flows through the full-intege... more Polar active matter of self-propelled particles sustain spontaneous flows through the full-integer topological defects. We study theoretically the incompressible flow profiles around ±1 defects induced by polar and dipolar active...
We adapt the Halperin–Mazenko formalism to analyze two-dimensional active nematics coupled to a g... more We adapt the Halperin–Mazenko formalism to analyze two-dimensional active nematics coupled to a generic fluid flow. The governing hydrodynamic equations lead to evolution laws for nematic topological defects and their corresponding density fields. We find that ±1/2 defects are propelled by the local fluid flow and by the nematic orientation coupled with the flow shear rate. In the overdamped and compressible limit, we recover the previously obtained active self-propulsion of the +1/2 defects. Non-local hydrodynamic effects are primarily significant for incompressible flows, for which it is not possible to eliminate the fluid velocity in favor of the local defect polarization alone. For the case of two defects with opposite charge, the non-local hydrodynamic interaction is mediated by non-reciprocal pressure-gradient forces. Finally, we derive continuum equations for a defect gas coupled to an arbitrary (compressible or incompressible) fluid flow.
The collective motion of driven or self-propelled interacting units is in many natural systems kn... more The collective motion of driven or self-propelled interacting units is in many natural systems known to produce complex patterns. This thesis considers two continuum field theories commonly used in describing pattern formation and dynamics: The first one, the phase field crystal model, which describes the dynamical and equilibrium properties of crystalline material, is used to study the coarsening dynamics of polycrystalline materials in two and three dimensions. A generalization introducing a faster elastic relaxation time scale is then used to study the plastic deformation and dislocation dynamics of single crystals. Secondly, a continuum theory describing mesoscopic turbulence of biological active matter, which is used to study long-range ordered vorticity patterns generated by cell divisions in a endothelial cell layer.
Abstract We perform an experimental analysis of stratified, co-current gas-liquid flow in a strai... more Abstract We perform an experimental analysis of stratified, co-current gas-liquid flow in a straight pipe, with the aim of investigating secondary flow structures in the gas-phase. Planar PIV (particle image velocimetry) of the pipe centerline as well as stereoscopic PIV of the pipe cross section is performed, considering cases where the interface is either rough (wavy) or flat. Results demonstrate that the interface morphology determines the direction of the secondary flow patterns. Over a flat interface, the secondary flow cells are directed up along the pipe centerline and down along the pipe walls. When the interface is rough (wavy), the direction of the secondary flow cells is reversed. Normalized by the peak axial velocity, the vertical velocity profile in the pipe centerline is seen to collapse to a common curve when the interface is flat, while results indicate that the normalized strength of the secondary flows increase with increasing surface roughness. An assessment of the Reynolds stresses in the cross section is performed, and the direction of the secondary flow cells are found to be well predicted by the dominant gradients of the radial and circumferential Reynolds stresses (τrr and τθθ) in the radial and circumferential direction respectively.
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