Emmanuel Lance Christopher Vi Medillo Plan

Models of active nematics in biological systems normally require complexity arising from the hydrodynamics involved at the microscopic level as well as the viscoelastic nature of the system. Here we show that a minimal, space-independent, model based on the temporal alignment of active and polymeric particles provides an avenue to predict and study their coupled dynamics within the framework of dynamical systems. In particular, we examine, using analytical and numerical methods, how such a simple model can display self-sustained oscillations in an activity-driven viscoelastic shear flow.

The success of Vietnam in controlling the spread of COVID-19 hinges on a timely implementation of its coherent strategy of containment and rapid tracing and testing efforts. The Vietnamese living in Mekong Delta are currently being besieged by the SARS-Cov-2 Delta variant as they undergo several and extended levels of lockdown. In this work we examine the temporal aspects of the lockdown in Ho Chi Minh City and predict the progress of the outbreak in terms of the total number of confirmed cases. A compartmental model with containment is fit to data to estimate the rate of transmission in Ho Chi Minh City. The severity of the lockdown is estimated from publicly-available data on mobility and coupled to the rate of infection. Various scenarios on when to begin a lockdown and its duration are assessed. This report, dated 27 July 2021, supports a lockdown of at least 3 weeks and predicts that there could be half as many cases had the inevitable lockdown started a week earlier.

Complex interactions between cellular systems and their surrounding extracellular matrices are emerging as important mechanical regulators of cell functions, such as proliferation, motility and cell death, and such cellular systems are often characterized by pulsating actomyosin activities. Here, using an active gel model, we numerically explore spontaneous flow generation by activity pulses in the presence of a viscoelastic medium. The results show that cross-talk between the activity-induced deformations of the viscoelastic surroundings and the time-dependent response of the active medium to these deformations can lead to the reversal of spontaneously generated active flows. We explain the mechanism behind this phenomenon based on the interaction between the active flow and the viscoelastic medium. We show the importance of relaxation time scales of both the polymers and the active particles and provide a phase space over which such spontaneous flow reversals can be observed. Our results suggest new experiments investigating the role of controlled pulses of activity in living systems ensnared in complex mircoenvironments.

Complex interactions between cellular systems and their surrounding extracellular matrices are emerging as important mechanical regulators of cell functions such as proliferation, motility, and cell death, and such cellular systems are often characterized by pulsating acto-myosin activities. Here, using an active gel model, we numerically explore the spontaneous flow generation by activity pulses in the presence of a viscoelastic medium. The results show that cross-talk between the activity-induced deformations of the viscoelastic surroundings with the time-dependent response of the active medium to these deformations can lead to the reversal of spontaneously generated active flows. We explain the mechanism behind this phenomenon based on the interaction between the active flow and the viscoelastic medium. We show the importance of relaxation timescales of both the polymers and the active particles and provide a phase-space over which such spontaneous flow reversals can be observed. Our results suggest new experiments investigating the role of controlled pulses of activity in living systems ensnared in complex mircoenvironments.

The 2019 Coronavirus Disease continues to spread, and in the interim absence of a proven cure and vaccine, preventive community interventions continue to be implemented across countries. Here we propose and analyze a minimal compartmental model that simultaneously incorporates three such interventions: lockdowns, rapid isolation, and institutional propaganda. Using a modified SEIRJ model, we vary three different factors in the quantity that determines the effective rate of infection. Estimates of the model parameters are obtained from publicly-available data and were used to simulate various combinations of interventions. Our results show that a combination of interventions could decrease the total number of infected individuals by one or two orders of magnitude and keep the number of active cases low. In particular we show that lockdowns need not be very stringent if proper social distancing via media awareness and rapid identification and isolation is practiced. We also highlight that an effective isolation system strongly improves the effect of these interventions.

Active matter systems such as eukaryotic cells and bacteria continuously transform chemical energy to motion. Hence living systems exert active stresses on the complex environments in which they reside. One recurring aspect of this complexity is the viscoelasticity of the medium surrounding living systems: Bacteria secrete their own viscoelastic extracellular matrix, and cells constantly deform, proliferate, and self-propel within viscoelastic networks of collagen. It is therefore imperative to understand how active matter modifies, and gets modified by, viscoelastic fluids. Here we present a two-phase model of active nematic matter that dynamically interacts with a passive viscoelastic polymeric phase and perform numerical simulations in two dimensions to illustrate its applicability. Motivated by recent experiments we first study the suppression of cell division by a viscoelastic medium surrounding the cell. We further show that the self-propulsion of a model keratocyte cell is modified by the polymer relaxation of the surrounding viscoelastic fluid in a nonuniform manner and find that increasing polymer viscosity effectively suppresses the cell motility. Last, we explore the hampering impact of the viscoelastic medium on the generic hydrodynamic instabilities of active nematics by simulating the dynamics of an active stripe within a polymeric fluid. The model presented here can provide a framework for investigating more complex dynamics such as the interaction of multicellular growing systems with viscoelastic environments.