arXiv subjects

Ranjiangshang Ran

Publications and source records attributed to Ranjiangshang Ran.

9 recordsLinked to original sources

arXiv2025

A microstructural rheological model for transient creep in polycrystalline ice

The slow creep of glacial ice plays a key role in sea-level rise, yet its transient deformation remains poorly understood. Glen's flow law, where strain rate is simply a function of stress, cannot predict the time-dependent creep behavior observed in experiments. Here we present a physics-based rheological model that captures all three regimes of transient creep in polycrystalline ice. The key components of the model are a series of Kelvin-Voigt mechanical elements that produce a power-law (Andrade) creep, and a single viscous element with microstructure and stress dependence that represents reorientation in the polycrystalline grains. The interplay between these components produces a minimum in the strain rate at approximately 1% strain, which is a universal but unexplained feature reported in experiments. Due to its transient nature, the model exhibits fractional power-law exponents in the stress dependence of the strain rate minimum, which has been conventionally interpreted as independent physical processes. Taken together, we provide a compact, mechanistic framework for transient ice rheology that generalizes to other polycrystalline materials and can be integrated into constitutive laws for ice-sheet models.

physics.geo-ph↗

arXiv2024

Enhancing Transport Barriers with Swimming Microorganisms in Chaotic Flows

We investigate the effects of bacterial activity on the mixing and transport properties of a passive scalar in time-periodic flows in experiments and in a simple model. We focus on the interactions between swimming E. coli and the Lagrangian Coherent Structures (LCSs) of the flow, which are computed from experimentally measured velocity fields. Experiments show that such interactions are non-trivial and can lead to transport barriers through which the scalar flux is significantly reduced. Using the Poincaré map, we show that these transport barriers coincide with the outermost members of elliptic LCSs known as Lagrangian vortex boundaries. Numerical simulations further show that elliptic LCSs can repel elongated swimmers and lead to swimmer depletion within Lagrangian coherent vortices. A simple mechanism shows that such depletion is due to the preferential alignment of elongated swimmers with the tangents of elliptic LCSs. Our results provide insights into understanding the transport of microorganisms in complex flows with dynamical topological features from a Lagrangian viewpoint.

physics.flu-dyn↗

arXiv2023

Sedimentation dynamics of passive particles in dilute bacterial suspensions: emergence of bioconvection

Microorganisms are ubiquitous in nature and technology. They inhabit diverse environments ranging from small river tributaries and lakes to oceans, as well as wastewater treatment plants and food manufacturing. In many of these environments, microorganisms coexist with settling particles. Here, we investigate the effects of microbial activity (swimming \textit{E. coli}) on the settling dynamics of passive colloidal particles using particle tracking methods. Our results reveal the existence of two distinct regimes in the correlation length scale ($L_u$) and the effective diffusivity of the colloidal particles ($D_{e\!f\!f}$), with increasing bacterial concentration ($ϕ_b$). At low $ϕ_b$, the parameters $L_u$ and $D_{e\!f\!f}$ monotonically increases with increasing $ϕ_b$. Beyond a critical $ϕ_b$, second regime is found in which both $D_{e\!f\!f}$ and $L_u$ are independent of $ϕ_b$. We demonstrate that the transition between these regimes is characterized by the emergence of bioconvection. We use experimentally-measured particle-scale quantities ($L_u$, $D_{e\!f\!f}$) to predict the critical bacterial concentration for the diffusion-bioconvection transition.

physics.flu-dyn↗

arXiv2022

Understanding the rheology of kaolinite clay suspensions using Bayesian inference

Mud is a suspension of fine-grained particles (sand, silt, and clay) in water. The interaction of clay minerals in mud gives rise to complex rheological behaviors, such as yield stress, thixotropy and viscoelasticity. Here, we experimentally examine the flow behaviors of kaolinite clay suspensions, a model mud, using steady shear rheometry. The flow curves exhibit both yield stress and rheological hysteresis behaviors for various kaolinite volume fractions ($ϕ_k$). Further understanding of these behaviors requires fitting to existing constitutive models, which is challenging due to numerous fitting parameters. To this end, we employ a Bayesian inference method, Markov chain Monte Carlo (MCMC), to fit the experimental flow curves to a microstructural viscoelastic model. The method allows us to estimate the rheological properties of the clay suspensions, such as viscosity, yield stress, and relaxation time scales. The comparison of the inherent relaxation time scales suggests that kaolinite clay suspensions are strongly viscoelastic and weakly thixotropic at relatively low $ϕ_k$, while being almost inelastic and purely thixotropic at high $ϕ_k$. Overall, our results provide a framework for predictive model fitting to elucidate the rheological behaviors of natural materials and other structured fluids.

cond-mat.soft↗

arXiv2022

Phase-separation during sedimentation of dilute bacterial suspensions

Numerous natural systems depend on the sedimentation of passive particles in presence of swimming microorganisms. Here, we investigate the dynamics of the sedimentation of spherical colloids at various E. coli concentration within the dilute regime. Results show the appearance of two sedimentation fronts, a spherical particle front and the bacteria front. We find that the bacteria front behave diffusive at short times, whereas at long times decays linearly. The sedimentation speed of passive particles decays at a constant speed and decreases as bacteria concentration ($ϕ_b$) is increased. As $ϕ_b$ is increased further, the sedimentation speed becomes independent of $ϕ_b$. The timescales of the bacteria front is associated with the particle settling speeds. Remarkably, all experiments collapse onto a single master line by using the bacteria front timescale. A phenomenological model is proposed that captures the sedimentation of passive particles in active fluids.

cond-mat.soft↗

arXiv2022

Mixing in chaotic flows with swimming bacteria

This is a manuscript accepted for publication on Physical Review Fluids, Gallery of Fluid Motion special issue. The manuscript is associated with a poster winner of the 39th Annual Gallery of Fluid Motion Award, for work presented at the 74th Annual Meeting of the American Physical Society's Division of Fluid Dynamics (Phoenix, AZ, USA 2021).

physics.flu-dyn↗

arXiv2022

Polymers in Swarming Bacterial Turbulence

We experimentally investigate the effects of polymer additives on the collective dynamics of swarming Serratia marcescens in quasi two-dimensional (2D) liquid films. We find that even minute amounts of polymers (< 20 ppm) can significantly enhance swimming speed and promote largescale coherent structures. Velocity statistics show that polymers suppress large velocity fluctuation, transforming the velocity distributions from super-Gaussian to Gaussian. Spatial and temporal correlation functions suggest that polymers increase both the size and lifetime of flow structures. The energy spectra show an exponential decay at low wavenumbers, with a characteristic length scale increasing with polymer concentration. Overall, these result show polymers can mediate bacteria interaction and promote large-scale coherence in dense active suspensions.

cond-mat.soft↗

arXiv2022

Flow and aerosol dispersion from wind musical instruments

In the midst of the COVID-19 pandemic, many live musical activities had to be postponed and even canceled to protect musicians and audience. Orchestral ensembles face a particular challenge of contamination because they are personnel heavy and instrumentally diverse. A chief concern is whether wind instruments are vectors of contamination through aerosol dispersion. This study, made possible by the participation of members of the Philadelphia Orchestra, brings insight on the modes of production and early life of aerosols of human origin emitted by wind instruments. We find that these instruments produce aerosol levels that are comparable to normal speech in quantity and size distribution. However, the exit jet flow speeds are much lower than violent expiratory events (coughing, sneezing). For most wind instruments, the flow decays to background indoor-air levels at approximately 2 meters away from the instrument's opening. Long range aerosol dispersion is thus via ambient air currents.

physics.flu-dyn↗

arXiv2021

Bacteria hinder large-scale transport and enhance small-scale mixing in time-periodic flows

Understanding mixing and transport of passive scalars in active fluids is important to many natural (e.g. algal blooms) and industrial (e.g. biofuel, vaccine production) processes. Here, we study the mixing of a passive scalar (dye) in dilute suspensions of swimming Escherichia coli in experiments using a two-dimensional (2D) time-periodic flow and in a simple simulation. Results show that the presence of bacteria hinders large scale transport and reduce overall mixing rate. Stretching fields, calculated from experimentally measured velocity fields, show that bacterial activity attenuates fluid stretching and lowers flow chaoticity. Simulations suggest that this attenuation may be attributed to a transient accumulation of bacteria along regions of high stretching. Spatial power spectra and correlation functions of dye concentration fields show that the transport of scalar variance across scales is also hindered by bacterial activity, resulting in an increase in average size and lifetime of structures. On the other hand, at small scales, activity seems to enhance local mixing. One piece of evidence is that the probability distribution of the spatial concentration gradients is nearly symmetric with a vanishing skewness. Overall, our results show that the coupling between activity and flow can lead to nontrivial effects on mixing and transport.

physics.flu-dyn↗