**6.1 Mixing**

*Digestive System - Recent Advances*

The advancing LLS and CC lead to the generation of pressure field and shear stress of similar trend. Local variations in the pressure along the axis indicate a linear variation in the non-contraction region and a nonlinear variation in the contraction region; zero at the center and boundaries—inlet and outlet of the intestinal segment. The pressure peaks at an offset from the center and shows symmetry about the axis for a contraction wave at the mid-segment. The wall shear stress shows a peak at the center of the contraction region and reduces to lower value at the either end of the wave and remains constant throughout the non-contraction region. Axial variations in the pressure and wall shear stress are similar for fluid of pseudo-plastic, Newtonian, and dilatants type. The study also reports that the pressure developed is higher for shear thickening fluids in comparison to shear-thinning fluid (**Table 4**). In a similar manner, wall shearing is highest for the dilatants. Shear stress in the lumen is highest at the wall and reduces linearly to the lowest value

**rate**

change

Wavelength ↑ or ↓ ↓ ↑ ↑ Effect of motility Occlusion ↑ ↑ ↑ ↑ Effect of motility

*Effect of contractility and rheology (normalized values) on flow; based on semi-analytical method.*

↑ ↑ ↓ ↓ Captures the

**↓** ↓ ↑ or ↓ ↑ or ↓ CC and LLS

**Peak luminal velocity**

**Physiological relevance**

behavior

rheology of diverse fluids

coordination

No change Extent of viscous

*Effect of simultaneous circular and longitudinal contractions on flow.*

**Parameter (↑) Pressure Shear stress Flow** 

Viscosity ↑ ↑ No

**60**

**Table 4.**

Flow behavior index

LLS spacing (about optimal)

**Figure 7.**

Using imaginary tracers, the author was able to determine particle trajectories due to the peristalsis—CC and LLS [56]. Two kinds of flows were observed; one resulting in axial displacement of the fluid and other causing circulation of the fluids (eddies). The radial displacement brought the fluid from the core region to the periphery and vice versa; thus allowing for flushing of the fluid proximal to the mucosa. However, the particles were displaced when the wave traverses the segment. Particle motion is highly dependent on the type of intestinal motility. Positioning of the tracers at various depths of the lumen showed different trajectory and followed the wall; particles close to the wall tend to follow the wall, while those near the axis exhibited near circulation. The authors report that the radial dimension of the whorls is found to be higher when the particles were positioned close to the wall and least at the center. Suggesting that, the contractions are more effective near the wall since the particles experience most of the wall momentum and least at the center of the lumen. Such a behavior is indicative of the mixing of the contents; given that the shearing is effective near the wall with formation of eddies.
