We study experimentally the motion of non-deformable microbeads in a linear shear flow close to a wall bearing a thin and soft polymer layer. Combining microfluidics and 3D optical tracking, we demonstrate that the steady-state bead/surface distance increases with the flow strength. Moreover, such lift is shown to result from flow-induced deformations of the layer, in quantitative agreement with theoretical predictions from elastohydrodynamics. This study thus provides the first experimental evidence of "soft lubrication" at play at small scale, in a system relevant e.g. to the physics of blood microcirculation.

Cell-cell and cell-glycocalyx interactions under flow are important for the behaviour of circulating cells in blood and lymphatic vessels. However, such interactions are not well understood due in part to a lack of tools to study them in defined environments. Here, we develop a versatile in vitro platform for the study of cell-glycocalyx interactions in well-defined physical and chemical settings under flow. Our approach is demonstrated with the interaction between hyaluronan (HA, a key component of the endothelial glycocalyx) and its cell receptor CD44. We generate HA brushes in situ within a microfluidic device, and demonstrate the tuning of their physical (thickness and softness) and chemical (density of CD44 binding sites) properties using characterisation with reflection interference contrast microscopy (RICM) and application of polymer theory. We highlight the interactions of HA brushes with CD44-displaying beads and cells under flow. Observations of CD44+ beads on a HA brush with RICM enabled the 3-dimensional trajectories to be generated, and revealed interactions in the form of stop and go phases with reduced rolling velocity and reduced distance between the bead and the HA brush, compared to uncoated beads. Combined RICM and bright-field microscopy of CD44+ AKR1 T-lymphocytes revealed complementary information about the dynamics of cell rolling and cell morphology, and highlighted the formation of tethers and slings, as they interacted with a HA brush under flow. This platform can readily incorporate more complex models of the glycocalyx, and should permit the study of how mechanical and biochemical factors are orchestrated to enable highly selective blood cell-vessel wall interactions under flow.