arXiv subjects

Rohan Vernekar

Publications and source records attributed to Rohan Vernekar.

7 recordsLinked to original sources

arXiv2026

Critical scaling and soft granular rheology of frictionless non-Brownian suspensions across jamming

We carry out 3D computer simulations to study suspension rheology across the jamming transition for deformable non-Brownian frictionless particles. The simulations are carried out at constant shear rates and suspension densities ($\dot{\gamma} , \phi$), for polydisperse spherical particles. The suspension is probed at densities in the neighbourhood of the jamming point $\phi_c$ , as well as across much wider density ranges. We carry out critical scaling analysis of our close-to-jamming data and confirm that shear-driven jamming is a second-order critical phenomenon requiring strong corrections on shear. We build maps of the local strain rate exponent to classify flow regimes and examine the effect of the corrections on the suspension rheology. Further, using the critical scaling jamming density, we carry out soft granular rheological scaling over the wider density range, demonstrating an excellent data collapse. The soft granular rheology relations are able to predict our data over almost the entire dynamical and density range.

cond-mat.soft↗

arXiv2026

Energetics, shearing and pumping efficiency of propagating contractions over villi-patterned wall

Intestinal villi undergo pendular-wave motility -- an active, propagating tissue motion driven by underlying longitudinal muscles. This motility drives irreversible, counter-wave fluid pumping, akin to the antiplectic metachrony of ciliary carpets, and generates a viscous mixing boundary layer above the villi tips, whose height is controlled by flow inertia. Using a simplified 2D model of the rat duodenum, we quantify the system's viscous energy dissipation and axial pumping efficiency. In contrast to the classical Stokes' second problem, we show that the fluid volume dominating energy dissipation is dictated by the intervillous geometry, remaining insensitive to the dynamically varying viscous mixing boundary layer height. The computed pumping efficiency is orders of magnitude lower than that of canonical peristalsis for equivalent flux pumping. We thus infer that bulk fluid pumping is not the primary biophysical function of propagating pendular-wave motility; instead, we postulate that its main role is to shear the mucus barrier layer over the villi-lined mucosa. Comparing the strain rate in the barrier region with canonical peristaltic reference values for a villi-free wall strongly supports our hypothesis. Finally, for biomimetic microfluidic applications, geometric optimization reveals that pumping efficiency scales quadratically with the channel-to-villi height ratio in Stokes flow, whereas in the inertial regime, dynamic flux confinement renders this geometric optimization strategy redundant.

physics.flu-dyn↗

arXiv2025

Intestinal villi and crypts density maximizing nutrient absorption

The villi and crypts of the gastrointestinal tract increase the effective surface area of the intestinal mucosa, potentially enhancing nutrient absorption. It is commonly assumed that this is their primary function, and that a higher villi density necessarily leads to improved absorption. However, when villi are packed too closely together, diffusion can be hindered, potentially offsetting this benefit. In this work, we investigate the relationship between the density of these structures and the overall efficiency of absorption. In three different simplified geometries, approximating crypts, leaf-like villi, and finger-like villi we calculate analytically the concentration profile and the absorption flux, assuming that there is only diffusion between these structures while the lumen is well mixed. When plotting the absorption flux per unit of gut length as a function of the structures' density, we observe that there is a density maximizing absorption. We study numerically this optimum. It depends weakly on the absorption properties of the given nutrient, so that a geometry optimal for one nutrient is close to optimum for another nutrient. Physiological data from various animal species align with this predicted optimal range and potentially reflect evolutionary selection for efficient nutrient uptake, supporting the model's validity.

q-bio.TO↗

arXiv2025

Hydrodynamics in a villi-patterned channel due to pendular-wave activity

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 between active microstructure, pendular-wave and finite inertia in biological flows, and suggests new avenues for flow control in biomimetic and microfluidic systems.

physics.flu-dyn↗

arXiv2023

The slow viscous flow around doubly-periodic arrays of infinite slender cylinders

The slow viscous flow through a doubly-periodic array of cylinders does not have an analytical solution. However, as a reduced model for the flow within fibrous porous media, this solution is important for many real-world systems. We asymptotically determine the flow around a doubly-periodic array of infinite slender cylinders, by placing doubly-periodic two-dimensional singularity solutions within the cylinder and expanding the no-slip condition on the cylinder's surface in powers of the cylinder radius. The asymptotic solution provides a closed-form estimate for the flow and forces as a function of the radius and the dimensions of the cell. The force is compared to results from lattice-Boltzmann simulations of low-Reynolds-number flows in the same geometry, and the accuracy of the no-slip condition on the surface of the cylinder, predicted by the asymptotic theory, is checked. Finally, the behaviour of the flow, flux, force and effective permeability of the cell is investigated as a function of the geometric parameters. The structure of the asymptotic permeability is consistent with other models for the flow parallel to an array of rods. These models could be used to help understand the flows within porous systems composed of fibres and systems involving periodic arrays such as deterministic lateral displacement.

physics.flu-dyn↗

arXiv2017

Anisotropic permeability in deterministic lateral displacement arrays

We uncover anisotropic permeability in microfluidic deterministic lateral displacement (DLD) arrays. A DLD array can achieve high-resolution bimodal size-based separation of microparticles, including bioparticles, such as cells. For an application with a given separation size, correct device operation requires that the flow remains at a fixed angle to the obstacle array. We demonstrate via experiments and lattice-Boltzmann simulations that subtle array design features cause anisotropic permeability. Anisotropic permeability indicates the microfluidic array's intrinsic tendency to induce an undesired lateral pressure gradient. This can cause an inclined flow and therefore local changes in the critical separation size. Thus, particle trajectories can become unpredictable and the device useless for the desired separation task. Anisotropy becomes severe for arrays with unequal axial and lateral gaps between obstacle posts and highly asymmetric post shapes. Furthermore, of the two equivalent array layouts employed with the DLD, the rotated-square layout does not display intrinsic anisotropy. We therefore recommend this layout over the easier-to-implement parallelogram layout. We provide additional guidelines for avoiding adverse effects of anisotropy on the DLD.

physics.flu-dyn↗

arXiv2015

Breakdown of deterministic lateral displacement efficiency for non-dilute suspensions: a numerical study

We investigate the effect of particle volume fraction on the efficiency of deterministic lateral displacement (DLD) devices. DLD is a popular passive sorting technique for microfluidic applications. Yet, it has been designed for treating dilute suspensions, and its efficiency for denser samples is not well known. We perform 3D simulations based on the immersed-boundary, lattice-Boltzmann and finite-element methods to model the flow of red blood cells (RBCs) in different DLD devices. We quantify the DLD efficiency in terms of appropriate "failure" probabilities and RBC counts in designated device outlets. Our main result is that the displacement mode breaks down upon an increase of RBC volume fraction, while the zigzag mode remains relatively robust. This suggests that the separation of larger particles (such as white blood cells) from a dense RBC background is simpler than separating smaller particles (such as platelets) from the same background. The observed breakdown stems from non-deterministic particle collisions interfering with the designed deterministic nature of DLD devices. Therefore, we postulate that dense suspension effects generally hamper efficient particle separation in devices based on deterministic principles.

cond-mat.soft↗