Post-doctorat : Etude des propriétés rhéologiques des suspensions concentrées et déformables en DEM

The small intestine is a complex organ with a dense neural feedback network to drive motility of smooth muscles in its walls for mixing, digesting and absorbing food. On its inner mucosa it posses a dense layer of elongated micron-sized structures called the villi, that significantly increase the surface area of the intestinal lumen for enhanced absorption¹ (fig.1). What is now well known is that the villi also undergo complex movement patterns due to the contraction of smooth longitudinal muscles, termed as pendular motion, the purpose of which remains unknown.

We carry out 2D simulations based on an idealised model of wall motility, taking inspiration from the complex motility patterns mapped from the duodenum of rats. We use the lattice-Boltzmann method (LBM) with moving boundaries, to understand patterns of flow, transport and mixing transfer within the intestinal lumen, taking into account the mechanical forcing exerted by the villi. We focus on two idealised mathematical motility models; namely propagating and stationary contractions. In the case of propagating contractions, each villus oscillates with a velocity that has a fixed phase lag ∆ϕ relative to its neighbour villi. On the other hand, for stationary longitudinal contractions, individual villi oscillation is independent of its neighbours, and depends solely on its starting position² .

In both types of contractions, the relative motion between adjacent villi induces cyclical fluid flux from the wall into the lumen and vice-versa. This radial “pumping” is determined by the variations in volume between adjacent villi. Oscillating boundaries pushing on a viscous fluid can cause steady streaming³ , which is nothing but the irreversible flow generated due to finite fluid inertia by time-periodic forcing. In the cases with standing contractions we find that steady streaming flow remains negligibly small to influence transport in the gut.

However, for propagating contractions, steady streaming flow can become large enough to generate a permanent irreversible flux at the centre of the intestinal lumen (fig.1). This flux depends on fluid inertia as well as the phase lag ∆ϕ between adjacent villi. This also generates surface fluxes close to the villi, that appear to advect intestinal contents along the mucosal wall. Therefore, propagating contractions generate axial transport of fluid in addition to radial “pumping”, which would enhance the absorption by locally spreading the nutrients over intestinal walls.

Figure 1: Left panel shows microgram of small intestinal villi of a brushtail possum1. The contour plots on the right show the steady streaming (irreversible) flow for two different phase lags for propagating contractions. The colour bars in both show velocity magnitude normalised by the maximum villi velocity.

¹R. G. Lentle, et al., Neurogastroenterology & Motility, 25, pp. 881-e700, (2013).

²Y. F. Lim, et al., Food Funct., 6, pp. 1787–1795, (2015).

³M. Puthumana Melepattu and C. de Loubens, Physics of Fluids, 34, p. 061905, (2022)