Dynamic states of swimming bacteria in a nematic liquid crystal cell with homeotropic alignment

Abstract

Flagellated bacteria such as Escherichia coli and Bacillus subtilis exhibit effective mechanisms for swimming in fluids and exploring the surrounding environment. In isotropic fluids such as water, the bacteria change swimming direction through the run-and-tumble process. Lyotropic chromonic liquid crystals (LCLCs) have been introduced recently as an anisotropic environment in which the direction of preferred orientation, the director, guides the bacterial trajectories. In this work, we describe the behavior of bacteria B. subtilis in a homeotropic LCLC geometry, in which the director is perpendicular to the bounding plates of a shallow cell. We demonstrate that the bacteria are capable of overcoming the stabilizing elastic forces of the LCLC and swim perpendicularly to the imposed director (and parallel to the bounding plates). The effect is explained by a finite surface anchoring of the director at the bacterial body; the role of surface anchoring is analyzed by numerical simulations of a rod realigning in an otherwise uniform director field. Shear flows produced by a swimming bacterium cause director distortions around its body, as evidenced both by experiments and numerical simulations. These distortions contribute to a repulsive force that keeps the swimming bacterium at a distance of a few micrometers away from the bounding plates. The homeotropic alignment of the director imposes two different scenarios of bacterial tumbling: one with an 180°r eversal of the horizontal velocity and the other with the realignment of the bacterium by two consecutive 90°turns. In the second case, the angle between the bacterial body and the imposed director changes from 90°to 0°and then back to 90°; the new direction of swimming does not correlate with the previous swimming direction. with a relatively flat rigid polyaromatic core and polar groups at the periphery [11][12][13]. In water, these molecules aggregate face-to-face in order to minimize the areas of unfavorable contact with water. Unlike their surfactantbased micellar and thermotropic counterparts, the LCLCs are not toxic to biological organisms [14].Recent experiments demonstrate that the prevailing direction of swimming is parallel to the directorn, i.e. to the average direction of LCLC orientation ( º -n n, = |ˆ| n 1) [7,8]. The orientational order of the LCLC environment can be controlled by temperature, the concentration of liquid crystal organic molecules, external electromagnetic fields and by surface alignment of the director [9,11,12]. The dispersion of swimming bacteria in LCLC, also called a living liquid crystal [8], offers new opportunities to control the dynamic behavior of the bacteria.The studies of swimming bacteria in LCLCs have been performed mostly for sandwich-type cells, in which the LCLC is confined between two glass plates, with the director being uniformly aligned along a certain direction in the plane of the cell (planar alignment). It has been shown that rod-like flagellated bacteria prefer to swim along the director [7,8,15,16]. It is ass...

Polar jets of swimming bacteria condensed by a patterned liquid crystal

“…1 2 W  , where W is the surface anchoring strength(21). The value of W is expected to be on the order of…”

Elementary Flow Field Profiles of Micro-Swimmers in Weakly Anisotropic Nematic Fluids: Stokeslet, Stresslet, Rotlet and Source Flows

“…This is one of the rationale behind the control of the bacteria swimming direction by using patterned surface anchoring [56], similar to patterned anchoring in colloidal suspensions driven by liquid-crystal-enabled electrophoresis [57]. However, in principle, bacteria can change its direction and swim even perpendicular to the nematic director [58]. This means that, in principle, all orientations of force dipoles relative to the director field (as shown in Figure 3) are relevant to experimental applications.…”

Focus on bacterial mechanics

“…In individual cells, flagella enable bacteria to swim through diverse environments. Zhou et al [29] investigated the ability of bacteria to swim in an environment that contains a preferred orientation through the use of lyotropic chromonic liquid crystals. Here, B. subtilis swimming was guided by the directors that bias the orientation of the cell.…”