Steady streaming flow induced by active biological microstructures; application to small intestine villi

Abstract

Physiological transport of fluid at small scales is often achieved by microscopic active finger-like 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 finger-like. 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. Lastly, orders of magnitude showed that SSF should contribute to the transport of particles, such as bacteria or nano-particles, on a layer of a few hundred micrometers above the villi and on time scales of few minutes.

Large vortices from soft vibrations

“…First, these results will be important for biological fluids where it is common to observe a pulsation of the flow, or for oscillating boundaries which generate steady streaming patterns in soft environments. Examples of such streaming situations are flows within the alveoli of the lungs due to breathing (Kumar et al 2011), rectified flows due to intestinal villi (Puthumana Melepattu & de Loubens 2022) or flow induced by the vibration of otolith stones in fish ears (Kotas et al 2006). In medicine, acoustic streaming flows enhance the delivery of drugs or accelerate the healing of injuries.…”

“…Examples of such streaming situations are flows within the alveoli of the lungs due to breathing (Kumar et al. 2011), rectified flows due to intestinal villi (Puthumana Melepattu & de Loubens 2022) or flow induced by the vibration of otolith stones in fish ears (Kotas et al. 2006).…”

Development of a microfluidic device to observe dynamic flow around the villi generated by deformation of small intestinal tissue

“…Owing to its complexity, luminal flow contributes not only to the transportation, retention, and mixing of food 9 but also to the transportation of GB around the villi. Computer simulations of the luminal flow around the villi 10,11 have been performed; however, fully predicting luminal flow using them is challenging. Therefore, to study the flow around villi, direct observation of the SI in vivo is a reasonable approach.…”