Clément de Loubens

Inspired by small intestine motility, we investigate the flow induced by a propagating pendular wave along the walls of a channel lined with rigid, villi-like microstructures. The villi undergo harmonic axial oscillations with a phase lag relative to their neighbours, generating travelling patterns of intervillous contraction. Using two-dimensional lattice Boltzmann simulations, we resolve the flow within the villi zone and the lumen, sampling small to moderate Womersley numbers. We uncover a mixing boundary layer (MBL) just above the villi, composed of semi-vortical structures that travel with the imposed wave. In the lumen, an axial steady flow emerges, surprisingly oriented opposite to the wave propagation direction, contrary to canonical peristaltic flows. We attribute this flow reversal to the non-reciprocal trajectories of fluid trapped between adjacent villi and derive a geometric scaling law that captures its magnitude in the Stokes regime. The MBL thickness is found to depend solely on the wave kinematics given by intervillous phase lag in the low-inertia limit. Above a critical threshold, oscillatory inertia induces dynamic confinement, limiting the radial extent of the MBL and leading to non-monotonic behaviour of the axial steady flux. We further develop an effective boundary condition at the villus tips, incorporating both steady and oscillatory components across relevant spatial scales. This framework enables coarse-grained simulations of intestinal flows without resolving individual villi. Our results shed light on the interplay among active microstructure, pendular wave and finite inertia in biological flows, and suggests new avenues for flow control in biomimetic and microfluidic systems.

We investigated the rheological properties of suspensions of attractive microcapsules, which formed a weak gel at volume fractions ϕ as low as 0.1. These microcapsules, measuring 100 μm in diameter, were constructed with a droplet of positively charged chitosan solution protected by a membrane formed through the complexation of chitosan with a negatively charged surfactant. Iso-density matched suspensions were achieved by dispersing these microcapsules in silicone oils. Plate-plate rheometry revealed that these suspensions displayed a yield stress ranging from about 0.1 to 3 Pa for ϕ increasing from 0.1 to 0.5. At much higher stresses, the suspension viscosity was almost shear independent. Furthermore, these suspensions exhibited a frequency sweep signature akin to attractive colloidal suspensions, with a shear elastic modulus plateauing at low frequencies, indicative of an elastic microstructure within the suspensions. Remarkably, a degree of microstructural anisotropy, reminiscent of fragile matter, was evidenced by the transient fluidization of the suspension when the direction of applied stress, which remained below the yield stress, was reversed. Beyond the yield stress, both symmetric and asymmetric stress reversal experiments demonstrated that the suspension structure was influenced by the applied stress. It changed from an anisotropic and fragile network at low stress levels to a dispersion of isolated particles at high stress levels. We concluded that suspensions of attractive microcapsules could be classified as a fragile particulate gel, whose microstructure depends on the stress and its direction.

Mechanistic understanding of anorectal (patho)physiology is missing to improve the medical care of patients suffering from defaecation disorders. Our objective is to show that complex fluid dynamics modelling of video defaecography may open new perspectives in the diagnosis of defaecation disorders. Based on standard X-ray video defaecographies, we developed a bi-dimensional patient-specific simulation of the expulsion of soft materials, the faeces, by the rectum. The model quantified velocity, pressure and stress fields during the defaecation of a neostool with soft stool-like rheology for patients showing normal and pathological defaecatory function. In normal defaecation, the proximal–distal pressure gradient resulted from both the anorectal junction which formed a converging channel and the anal canal. The flow of the neostool through these anatomical parts was dominated by its shear-thinning viscous properties, rather than its yield stress. Consequently, the evacuation flow rate was significantly affected by variations in pressure applied by the rectum, and much less by the geometry of the anorectal junction. Lastly, we simulated impaired defaecations in the absence of obvious obstructive phenomena. Comparison with normal defaecation allowed us to discuss critical elements which should lead to effective medical management.

Physiological transport of fluid at small scales is often achieved by microscopic active fingerlike structures. It is recognized that they have to move in a non-symmetric fashion in order to break the symmetry of creeping flow and to induce a net movement of the fluid. However, in the limit of low, but non-vanishing, Reynolds number, irreversible flow on long time scales could also be generated by symmetric oscillations of these microstructures. Inspired by small intestine villi, we reported three dimensional direct numerical simulations of the irreversible part of the flow, namely steady streaming flow (SSF), generated by an array of oscillating fingerlike structures. In order to capture these second order flow phenomena, the algorithm was based on a combination of lattice-Boltzmann methods with two relaxation times and the smoothed profile method. SSF was confined inside a steady viscous boundary above the villi. Two steady vortices at the tip of the villi characterized this flow which induced mass transfers between the bulk and the periphery. Strikingly, the spatial extension of these vortices was not solely governed by the Stokes boundary layer but also by the lateral confinement between the villi. Moreover, secondary vortices outside the steady boundary layer were also observed. These findings were rationalized in a state diagram showing three regimes of SSF. Finally, orders of magnitude showed that SSF should contribute to the transport of particles, such as bacteria or nano-particles, on a layer a few hundred micrometers above the villi and on a time scale of few minutes.

Using absorbance measurements through a Couette cell containing an emulsion of buoyant droplets, volume fraction profiles are measured at various shear rates. These viscous resuspension experiments allow a direct determination of the normal stress in the vorticity direction in connection with the suspension balance model that has been developed for suspensions of solid particles. The results unambigously show that the normal viscosity responsible for the shear-induced migration of the droplets is independent on the capillary number, implying that particle deformation does not play a major role. It is similar to that of rigid particles at volume fractions below 40% but much smaller at higher ones.