How a single bacterium becomes a colony of many thousand cells is important in biomedicine and food safety. Much is known about the molecular and genetic bases of this process, but less about the underlying physical mechanisms. Here we study the growth of single-layer micro-colonies of rod-shaped Escherichia coli bacteria confined to just under the surface of soft agarose by a glass slide. Analysing this system as a liquid crystal, we find that growth-induced activity fragments the colony into microdomains of well-defined size, whilst the associated flow orients it tangentially at the boundary. Topological defect pairs with charges are produced at a constant rate, with the defects being propelled to the periphery. Theoretical modelling suggests that these phenomena have different physical origins from similar observations in other extensile active nematics, and a growing bacterial colony belongs to a new universality class, with features reminiscent of the expanding universe.
Abstract. -We investigate the transition between the Cassie-Baxter and Wenzel states of a slowly evaporating, micron-scale drop on a superhydrophobic surface. In two dimensions analytical results show that there are two collapse mechanisms. For long posts the drop collapses when it is able to overcome the free energy barrier presented by the hydrophobic posts. For short posts, as the drop loses volume, its curvature increases allowing it to touch the surface below the posts. We emphasise the importance of the contact line retreating across the surface as the drop becomes smaller: this often preempts the collapse. In a quasi-three dimensional simulation we find similar behaviour, with the additional feature that the drop can de-pin from all but the peripheral posts, so that its base resembles an inverted bowl.
We perform dynamical simulations of a two-dimensional active nematic fluid in coexistence with an isotropic fluid. Drops of active nematic become elongated, and an effective anchoring develops at the nematic-isotropic interface. The activity also causes an undulatory instability of the interface. This results in defects of positive topological charge being ejected into the nematic, leaving the interface with a diffuse negative charge. Quenching the active lyotropic fluid results in a steady state in which phase-separating domains are elongated and then torn apart by active stirring.Many biophysical nematic systems, including microtubule bundles [1], cytoskeletal filaments ordered by molecular motors in motility assays [2], actin filaments [3], cells [4,5] and dense suspensions of microswimmers [6] are active, meaning that the constituent particles generate motion by dissipating chemical energy, for example from adenosine tirphosphate [7,8]. This motion collectively manifests itself as a stress that keeps the system out of equilibrium. Active nematics exhibit rich pattern formation [9][10][11] and collective motion [12].Almost all studies of active nematics thus far have concentrated on bulk systems. However there are many examples where active material coexists with an isotropic fluid. These include active droplets [1,[12][13][14], biofilms, bacterial colonies and bacterial carpets [15][16][17]. Existing studies include spontaneous division and motility of active nematic droplets through self generated flows [13]. However, we are not aware of research reporting the phase separation of active fluids nor the role of topological defects in lyotropic active nematics.Therefore in this Letter we describe the behaviour of active nematic -isotropic mixtures. We show that active forces lead to nematic anchoring at the interface, as observed in growing bacterial colonies [18,19]. Moreover, active forces elongate nematic domains in the direction parallel (perpendicular) to the director field for extensile (contractile) systems. The elongated domains are torn apart by hydrodynamic instabilites, which balance the tendency to phase ordering, forming a dynamic steady state with characteristic length scales. Furthermore, we find that defect formation is dominated by the ejection of point defects with topological charge +1/2 from the interface, leaving the interface itself with a negative topological charge.The nematic order of the fluid is described by a symmetric, traceless tensor Q [20]. We assume that the director always remains within the plane of the system so Q αβ = S (2n α n β − δ αβ ) is two-dimensional, with n the director and S the magnitude of the order. The active nematic fluid is mixed with an isotropic fluid, and the amount of each is conserved. We use a scalar parameter φ to measure the relative density of each at a given point. The free energy of the system iswhere A, C, K and L are positive constants. The first term in f is the bulk energy of the binary fluid [21,22], which has two equilibria at φ = 0, 1. The s...
We present and interpret simulation results showing how a fluid moves on a hydrophilic substrate patterned by a square array of triangular posts. We demonstrate that the shape of the posts leads to anisotropic spreading, and discuss how this is influenced by the different ways in which the posts can pin the advancing front.
Understanding the flow of liquid crystals in microfluidic environments plays an important role in many fields, including device design and microbiology. We perform hybrid lattice-Boltzmann simulations of a nematic liquid crystal flowing under an applied pressure gradient in two-dimensional channels with various anchoring boundary conditions at the substrate walls. We investigate the relation between flow rate and pressure gradient and the corresponding profile of the nematic director, and find significant departures from the linear Poiseuille relation. We also identify a morphological transition in the director profile and explain this in terms of an instability in the dynamical equations. We examine the qualitative and quantitative effects of changing the type and strength of the anchoring. Understanding such effects may provide a useful means of quantifying the anchoring of a substrate by measuring its flow properties.
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