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Fluid mechanics at biological interfaces : from membranes to motile organisms
Biology is wet and active. On the micron-scale, the complex interactions at the interfaces between fluids and soft, deformable biological matter give rise to unexpected fluid mechanics. We use recently developed optical tools, from optical tweezers to multiview microscopy to unravel surprising fluid mechanics at biological interfaces, from biological membranes to motile organisms.
1. Starting at the smallest scale, we will first discuss the fluid mechanics of biological membranes. A cell’s interactions with the environment are mediated by its cellular membrane. This nanometer-thick, liquid crystalline structure is mostly composed of a lipid bilayer, which serves as a scaffold for embedded proteins and other macromolecules. In this study, we use optical tweezers to both apply and measure local forces on free-standing lipid bilayers within microfluidic channels. These measurements give insight into the boundary conditions at an interface between a fluid and a lipid bilayer.
2. We will then consider the fluid mechanics around motile ciliated cells. There, Stokes equations are commonly used to model the hydrodynamic flow around cilia. Using a new velocimetry methods, combining optical trapping and Kalman filtering, we investigate the validity of the zero Reynolds number approximation. We find that beating cilia generate a flow, which fundamentally differs from the stokeslet field predicted by Stokes equations. This indicates that the quasisteady approximation and use of Stokes equations for unsteady ciliary flow are not always justified and the finite timescale for vorticity diffusion cannot be neglected.
3. Considering interactions between motile unicellular organisms swimming through complex environments, they often interacting with solid surfaces. Their swimming is influenced by the proximity to solid substrates, through hydrodynamic and steric interactions. These interactions directly influence the cell population density distribution and the residence time in the vicinity of the surface. Swimming cells are recorded simultaneously by four separate cameras and triangulated in three-dimensions. Our results provide evidence of the existence of a long-range hydrodynamic interaction, which induces orbiting behaviour in the near-surface region.
4. If time allows, I will discuss recent rheological measurements of active suspensions. The shear viscosity of contractile suspensions of several volume fractions has been experimentally measured at different temperatures and shear rates. We show that temperature affects the rheology through the motility of the particles. As a result, the suspension shows unexpected rheological response to temperature such as `heat-thickening, whereby the stresses in the system increase with temperature. We find that motility of the particles can trigger a glass-like transition in the suspension due to the emergence of motility induced stresses that resembles a yield stress. Our study suggests that the emergence of this yield-like stress induced by motility is at the origin of the different rheological properties of the active suspension.