arXiv subjects

Yongfeng Zhao

Publications and source records attributed to Yongfeng Zhao.

13 recordsLinked to original sources

arXiv2026

Spontaneous currents determine capillary rise in active matter

The capillary rise of simple passive fluids in a tube is given by Jurin's law of capillary action, which balances surface tension and gravity. For fluids composed of active particles interacting via pairwise forces, a capillary rise was reported despite a negative mechanical surface tension, a phenomenon which remains unexplained. We establish the active form of Jurin's law from the microscopic dynamics. It includes a drag emerging from particle currents that we find responsible for capillary rise. These active currents, alongside negative surface tension, lead to complex and counterintuitive capillary action phenomena that are impossible in equilibrium. In particular the capillary rise of active fluids depends on the shape of the tube, not solely on the tube diameter.

cond-mat.soft↗

arXiv2025

Detecting active L\'evy particles using differential dynamic microscopy

Detecting L\'evy flights of cells has been a challenging problem in experiments. The challenge lies in accessing data in spatiotemporal scales across orders of magnitude, which is necessary for reliably extracting a power-law scaling. Differential dynamic microscopy has been shown to be a powerful method that allows one to acquire statistics of cell motion across scales, which is a potentially versatile method for detecting L\'evy walks in biological systems. In this article, we extend the differential dynamic microscopy method to self-propelled L\'evy particles, whose run-time distribution has an algebraic tail. We validate our protocol using synthetic imaging data and show that a reliable detection of active L\'evy particles requires accessing length scales of an order of magnitude larger than its persistence length, if the variability in particle speed is moderate. Applying the protocol to experimental data of E. coli and E. gracilis, we find that E. coli does not exhibit a signature of L\'evy walks, while E. gracilis is better described as active L\'evy particles.

cond-mat.soft↗

arXiv2024

Integer Topological Defects Reveal Anti-Symmetric Forces in Active Nematics

Cell layers are often categorized as contractile or extensile active nematics but recent experiments on neural progenitor cells with induced $+1$ topological defects challenge this classification. In a bottom-up approach, we first study a relevant particle-level model and then analyze a continuous theory derived from it. We show that both model and theory account qualitatively for the main experimental result, i.e. accumulation of cells at the core of any type of +1 defect. We argue that cell accumulation is essentially due to two generally ignored 'effective active forces'. We finally discuss the relevance and consequences of our findings in the context of other cellular active nematics experiments and previously proposed theories.

cond-mat.soft↗

arXiv2024

Active particles in a tube: a generalized entropy potential approach

We study the transport of self-propelled noninteracting active Brownian particles (ABPs) and run-and-tumble particles (RTPs) in long tubes of varying widths. Using a moment expansion, we construct a generalized Fick-Jacobs framework for the active particles when the tube width is large and slowly varying. We show that the variation of the particle density along the tube is well described by a one-dimensional generalized entropy potential. This potential resembles its passive counterpart, albeit with an effective temperature and an effective tube width that are renormalized by the activity. Our generalized entropy potential approach allows us to predict the steady-state density distribution along the tube as well as the mean escape time out of a spindle chamber. Finally, we show how to account for the emergence of spontaneous ratchet flows in asymmetric channel by including higher-order corrections neglected in the effective entropy potential approach.

cond-mat.soft↗

arXiv2024

Active Young-Dupré Equation: How Self-organized Currents Stabilize Partial Wetting

The Young-Dupré equation is a cornerstone of the equilibrium theory of capillary and wetting phenomena. In the biological world, interfacial phenomena are ubiquitous, from the spreading of bacterial colonies to tissue growth and flocking of birds, but the description of such active systems escapes the realm of equilibrium physics. Here we show how a microscopic, mechanical definition of surface tension allows us to build an Active Young-Dupré equation able to account for the partial wetting observed in simulations of active particles interacting via pairwise forces. Remarkably, the equation shows that the corresponding steady interfaces do not result from a simple balance between the surface tensions at play but instead emerge from a complex feedback mechanism. The interfaces are indeed stabilized by a drag force due to the emergence of steady currents, which are themselves a by-product of the symmetry breaking induced by the interfaces. These currents also lead to new physics by selecting the sizes and shapes of adsorbed droplets, breaking the equilibrium scale-free nature of the problem. Finally, we demonstrate a spectacular consequence of the negative value of the liquid-gas surface tensions in systems undergoing motility-induced phase separation: partially-immersed objects are expelled from the liquid phase, in stark contrast with what is observed in passive systems. All in all, our results lay the foundations for a theory of wetting in active systems.

cond-mat.soft↗

arXiv2022

Non-reciprocity across scales in active mixtures

In active matter, the lack of momentum conservation makes non-reciprocal interactions the rule rather than the exception. They lead to a rich set of emerging behaviors that are hard to account for and to predict starting from the microscopic scale, due to the absence of a generic theoretical framework out of equilibrium. Here we consider bacterial mixtures that interact via mediated, non-reciprocal interactions like quorum-sensing and chemotaxis. By explicity relating microscopic and macroscopic dynamics, we show that non-reciprocity may fade as coarse-graining proceeds, leading to large-scale bona fide equilibrium descriptions. In turns, this allows us to account quantitatively, and without fitting parameters, for the rich behaviors observed in microscopic simulations including phase separation, demixing or multi-phase coexistence. We also derive the condition under which non-reciprocity is strong enough to survive coarse-graining, leading to a wealth of dynamical patterns. Again, the explicit coarse-graining of the dynamics allows us to predict the phase diagram of the system starting from its microscopic description. All in all, our work demonstrates that the fate of non-reciprocity across scales is a subtle and important question.

cond-mat.stat-mech↗

arXiv2022

Quantitative characterization of run-and-tumble statistics in bulk bacterial suspensions

We introduce a numerical method to extract the parameters of run-and-tumble dynamics from experimental measurements of the intermediate scattering function. We show that proceeding in Laplace space is unpractical and employ instead renewal processes to work directly in real time. We first validate our approach against data produced using agent-based simulations. This allows us to identify the length and time scales required for an accurate measurement of the motility parameters, including tumbling frequency and swim speed. We compare different models for the run-and-tumble dynamics by accounting for speed variability at the single-cell and population level, respectively. Finally, we apply our approach to experimental data on wild-type Escherichia coli obtained using differential dynamic microscopy.

physics.bio-ph↗

arXiv2022

Characterization and Control of the Run-and-Tumble Dynamics of {\it Escherichia Coli}

We characterize the full spatiotemporal gait of populations of swimming {\it Escherichia coli} using renewal processes to analyze the measurements of intermediate scattering functions. This allows us to demonstrate quantitatively how the persistence length of an engineered strain can be controlled by a chemical inducer and to report a controlled transition from perpetual tumbling to smooth swimming. For wild-type {\it E.~coli}, we measure simultaneously the microscopic motility parameters and the large-scale effective diffusivity, hence quantitatively bridging for the first time small-scale directed swimming and macroscopic diffusion.

cond-mat.soft↗

arXiv2022

An Introduction to Motility-Induced Phase Separation

Active particles may undergo phase separation when interactions oppose self-propulsion, in the absence of any cohesive forces. The corresponding Motility-Induced Phase Separation (MIPS) is arguably the simplest non-trivial collective feature that distinguishes active from passive particles. It is observed in a large variety of systems which we review in this chapter. We describe in depth the case of motile particles interacting via quorum-sensing interactions, whose theoretical framework is by now well-established. We close the chapter by discussing the features observed in systems undergoing MIPS that still challenge our understanding.

cond-mat.soft↗

arXiv2021

Controlling cell motion and microscale flow with polarized light fields

We investigate how light polarization affects the motion of photo-responsive algae, \textit{Euglena gracilis}. In a uniformly polarized field, cells swim approximately perpendicular to the polarization direction and form a nematic state with zero mean velocity. When light polarization varies spatially, cell motion is modulated by local polarization. In such light fields, cells exhibit complex spatial distribution and motion patterns which are controlled by topological properties of the underlying fields; we further show that ordered cell swimming can generate directed transporting fluid flow. Experimental results are quantitatively reproduced by an active Brownian particle model in which particle motion direction is nematically coupled to local light polarization.

cond-mat.soft↗

arXiv2019

Surface Tensions between Active Fluids and Solid Interfaces: bare vs dressed

We analyze the surface tension exerted at the interface between an active fluid and a solid boundary in terms of tangential forces. Focusing on active systems known to possess an equation of state for the pressure, we show that interfacial forces are of a more complex nature. Using a number of macroscopic setups, we show that the surface tension is a combination of an equation-of-state abiding part and of setup-dependent contributions. The latter arise from generic setup-dependent steady currents which "dress" the measurement of the "bare" surface tension. The former shares interesting properties with its equilibrium counterpart, and can be used to generalize the Young-Laplace law to active systems. We finally show how a suitably designed probe can directly access this bare surface tensions, which can also be computed using a generalized Virial formula.

cond-mat.soft↗

arXiv2019

Activated escape of a self-propelled particle from a metastable state

We study the noise-driven escape of active Brownian particles (ABPs) and run-and-tumble particles (RTPs) from confining potentials. In the small noise limit, we provide an exact expression for the escape rate in term of a variational problem in any dimension. For RTPs in one dimension, we obtain an explicit solution, including the first sub-leading correction. In two dimensions we solve the escape from a quadratic well for both RTPs and ABPs. In contrast to the equilibrium problem we find that the escape rate depends explicitly on the full shape of the potential barrier, and not only on its height. This leads to a host of unusual behaviors. For example, when a particle is trapped between two barriers it may preferentially escape over the higher one. Moreover, as the self-propulsion speed is varied, the escape route may discontinuously switch from one barrier to the other, leading to a dynamical phase transition.

cond-mat.soft↗

arXiv2018

Optimized Diffusion of Run-and-Tumble Particles in Crowded Environments

We study the transport of self-propelled particles in dynamic complex environments. To obtain exact results, we introduce a model of run-and-tumble particles (RTPs) moving in discrete time on a $d$-dimensional cubic lattice in the presence of diffusing hard core obstacles. We derive an explicit expression for the diffusivity of the RTP, which is exact in the limit of low density of fixed obstacles. To do so, we introduce a generalization of Kac's theorem on the mean return times of Markov processes, which we expect to be relevant for a large class of lattice gas problems. Our results show the diffusivity of RTPs to be nonmonotonic in the tumbling probability for low enough obstacle mobility. These results prove the potential for optimization of the transport of RTPs in crowded and disordered environments with applications to motile artificial and biological systems.

cond-mat.stat-mech↗