In many situations, bacteria move in complex environments, as soils, oceans or the human gut-track, where carrier fluids show complex structures associated with non-Newtonian rheology. Many fundamental questions concerning the ability to navigate in such environments remain unsolved. Recently, it has been shown that the kinetics of bacterial motion in structured fluids as liquid crystals (LCs) is constrained by the orientational molecular order (or director field) and that novel spatio-temporal patterns arise. A question unaddressed so far is how bacteria change swimming direction in such an environment. In this work, we study the swimming mechanism of a single bacterium, Esherichia coli, constrained to move along the director field of a lyotropic chromonic liquid crystal confined to a planar cell. Here, the spontaneous ‘run and tumble’ motion of the bacterium gets frustrated: the elasticity of the LC prevents flagella from unbundling. Interestingly, to change direction, bacteria execute a reversal motion along the director field, driven by the relocation of a single flagellum, a ‘frustrated tumble’. We characterize this phenomenon in detail experimentally, exploiting exceptional spatial and temporal resolution of bacteria I and flagella r dynamics, using a two colour Lagrangian tracking technique. We suggest a possible mechanism accounting for these observations.
We examine experimentally the deformation of flexible, microscale helical ribbons with nanoscale thickness subject to viscous flow in a microfluidic channel. Two aspects of flexible microhelices are quantified: the overall shape of the helix and the viscous frictional properties. The frictional coefficients determined by our experiments are consistent with calculated values in the context of resistive-force theory. The deformation of helices by viscous flow is well described by nonlinear finite extensibility. Under distributed loading, the pitch distribution is nonuniform, and from this we identify both linear and nonlinear behavior along the contour length of a single helix. Moreover, flexible helices are found to display reversible global to local helical transitions at a high flow rate.
We experimentally investigate the growth dynamics of cavities nucleating during the first stages of debonding of three different model adhesives. The material properties of these adhesives range from a more liquid-like material to a soft viscoelastic solid and are carefully characterized by small strain oscillatory shear rheology as well as large strain uniaxial extension. The debonding experiments are performed on a probe tack set-up. Using high contrast images of the debonding process and precise image analysis tools we quantify the total projected area of the cavities, the average cavity shape and growth rate and link these observations to the material properties. These measurements are then used to access corrected effective stress and strain curves that can be directly compared to the results from the uniaxial extension.
We propose a novel device capable of measuring the relaxation time of viscoelastic fluids as small as 1\,ms. In contrast to most rheometers, which by their very nature are concerned with producing viscometric or nearly-viscometric flows, here we make use of an elastic instability which occurs in the flow of viscoelastic fluids with curved streamlines. To calibrate the rheometer we combine simple scaling arguments with relaxation times obtained from first normal-stress difference data measured in a classical shear rheometer. As an additional check we also compare these relaxation times to those obtained from Zimm theory and good agreement is observed. Once calibrated, we show how the serpentine rheometer can be used to access smaller polymer concentrations and lower solvent viscosities where classical measurements become difficult or impossible to use due to inertial and/or resolution limitations. In the absence of calibration the serpentine channel can still be a very useful comparative or index device.
We report experiments on the deformation and transport of an elastic fiber in a viscous cellular flow, namely a lattice of counter-rotative vortices. We show that the fiber can buckle when approaching a stagnation point. By tuning either the flow or fiber properties, we measure the onset of this buckling instability. The buckling threshold is determined by the relative intensity of viscous and elastic forces, the elasto-viscous number Sp. Moreover we show that flexible fibers escape faster from a vortex (formed by closed streamlines) compared to rigid fibers. As a consequence, the deformation of the fiber changes its transport properties in the cellular flow.
The flow of complex suspensions is ubiquitous in nature and industrial applications. These suspensions are made of complex particles (anisotropic, deformable, or active) suspended in simple fluids. The macroscopic non-Newtonian properties of these suspensions depend on the nature of the suspended particles and their interaction with given flows. Here, we describe how one can make use of novel micro-fabrication techniques and microfluidic rheometers to determine their flow properties under well controlled experimental conditions. We discuss three different aspects important for the study of the flow of complex suspensions. First, we use a well known complex fluid to design a novel microfluidic rheometer. Then we measure the non-Newtonian shear viscosity of a dilute suspension of microswimmers adapting an existing microfluidic rheometer. And finally we use a micro-fabrication technique to produce well controlled model fibers inside microfluidic channels and give some examples of the flow dynamics when these model fiber suspensions are forced through constrictions.
The interaction of flexible structures with viscoelastic flows can result in very rich dynamics. In this paper, we present the results of the interactions between the flow of a viscoelastic polymer solution and a cantilevered beam in a confined microfluidic geometry. Cantilevered beams with varying length and flexibility were studied. With increasing flow rate and Weissenberg number, the flow transitioned from a fore-aft symmetric flow to a stable detached vortex upstream of the beam, to a time-dependent unstable vortex shedding. The shedding of the unstable vortex upstream of the beam imposed a time-dependent drag force on the cantilevered beam resulting in flow-induced beam oscillations. The oscillations of the flexible beam were classified into two distinct regimes: a regime with a clear single vortex shedding from upstream of the beam resulting in a sinusoidal beam oscillation pattern with the frequency of oscillation increasing monotonically with Weissenberg number, and a regime at high Weissenberg numbers characterized by 3D chaotic flow instabilities where the frequency of oscillations plateaued. The critical onset of the flow transitions, the mechanism of vortex shedding and the dynamics of the cantilevered beam response are presented in detail here as a function of beam flexibility and flow viscoelasticity.
We propose a novel device capable of measuring relaxation times of viscoelastic fluids as small as 1 ms. In contrast to most rheometers, which by their very nature are concerned with producing viscometric or nearly-viscometric flows, here we make use of an elastic instability that occurs in the flow of viscoelastic fluids with curved streamlines. To calibrate the rheometer we combine simple scaling arguments with relaxation times obtained from first normal-stress difference data measured in a classical shear rheometer. As an additional check we also compare these relaxation times to those obtained from Zimm theory and good agreement is observed. Once calibrated, we show how the serpentine rheometer can be used to access smaller polymer concentrations and lower solvent viscosities where classical measurements become difficult or impossible to use due to inertial and/or resolution limitations. In the absence of calibration, the serpentine channel can still be a very useful comparative or index device.
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