arXiv subjects

María Luisa Cordero

Publications and source records attributed to María Luisa Cordero.

7 recordsLinked to original sources

arXiv2025

Soil-mimicking microfluidic devices reveal restricted flagellar motility of Bradyrhizobium diazoefficiens under microconfinement

Bradyrhizobium diazoefficiens is a nitrogen-fixing symbiont of soybean, worldwide used as biofertilizer. This soil bacterium possesses two flagellar systems enabling it to swim in water-saturated soils. However, the motility in soil pores, which may be crucial for competitiveness for root nodulation, is difficult to predict. To address this gap, we fabricated microfluidic devices with networks of connected microchannels surrounding grains. In them, we directly visualise bacterial behaviour in transparent geometries mimicking minimalist soils-on-a-chip (SOCs). We measured the population velocities and changes of direction for two strains: the wild-type and a mutant with only a subpolar flagellum. A detailed statistical analysis revealed that both strains exhibited reduced speeds and increased changes of direction of 180$^\circ$, in channels of decreasing cross sectional area, down to a few microns in width. Interestingly, while the wild-type strain displayed faster swimming in unconfined spaces, this advantage was negated in the SOCs with the narrowest microchannels. We employed the measured motility parameters to propose a realistic model and simulate B. diazoefficiens confined dynamics being able to reproduce their behaviour, which additionally can be extended enabling further predictions for long time and macro scales. This multidisciplinary work, combining design, microfabrication, microbiology and modelling, offers useful methods to study soil bacteria and may be readily extended to other beneficial/harmful soil species.

physics.bio-ph↗

arXiv2025

Active bacterial baths in droplets

Suspensions of self-propelled objects represent a novel paradigm in colloidal science. In such active baths traditional concepts, such as Brownian motion, fluctuation-dissipation relations, and work extraction from heat reservoirs, must be extended beyond the conventional framework of thermal baths. Unlike thermal baths, which are characterized by a single parameter, the temperature, the fundamental descriptors of an active bath remain elusive, especially in confined environments. In this study, buoyant, passive tracers are employed as generalized probes to investigate an active bath comprising motile bacteria confined within a droplet. We demonstrate that momentum transfer from the bath to the tracer can be effectively described as colored noise, characterized by temporal memory and an enhanced effective diffusivity significantly larger compared to thermal Brownian motion values. Using a stochastic analytical framework, we extract the temporal memory and diffusivity parameters that define such an active bath. Notably, the diffusivity scales linearly with bacterial concentration, modulated by a factor representing the role of confinement, expressed as the ratio of the confining radius to the probe radius. This finding, while still awaiting a complete theoretical explanation, offers new insights into the transport properties of confined active baths and paves the way for a deeper understanding of active emulsions driven by confined active matter.

cond-mat.soft↗

arXiv2025

Damage and recovery of flagella in soil bacteria exposed to shear within long microchannels

The swimming motility of bacteria is driven by the action of bacterial flagellar motors, whose outermost structure is a long and thin helicoidal filament. When rotated, the fluid medium exerts an anisotropic viscous drag on the flagellar filaments, ultimately leading to bacterial propulsion. The flagellar filaments are protein-based flexible structures that can break due to interactions with fluid flows. Here, we study the evolution of flagellar filaments in the soil bacterium $\textit{Bradyrhizobium diazoefficiens}$ after being exposed to shear flows created in long microchannels, for shear rates between $1$ s$^{-1}$ and $10^5$ s$^{-1}$, and for durations between tens of milliseconds and minutes. We demonstrate that the average swimming speed and fraction of swimming cells decrease after exposition to shear, but both parameters can recover, at least partially, with time. These observations support the hypothesis that shear flows cut flagellar filaments but that reversibly damaged bacterial flagellar motors can be restored thanks to filament regeneration. By fitting our observations with phenomenological expressions, we obtain the individual growth rates of the two different flagellar filaments that $\textit{B. diazoefficiens}$ possesses, showing that the lateral filaments have a recovery time of about 40 min while the subpolar one requires more than 4.5 h to regrow. Our work demonstrates that simple monitoring of bacterial motility after exposition to shear can be used to characterize the process of flagellar filament breakup and growth, a phenomenon widely present in bacteria swimming in porous soil and exposed to shear flows due to rainfall and watering systems.

physics.bio-ph↗

arXiv2024

Recovering the activity parameters of an active fluid confined in a sphere

The properties of an active fluid, for example, a bacterial bath or a collection of microtubules and molecular motors, can be accessed through the dynamics of passive particle probes. Here, in the perspective of analyzing experimental situations of confinement in droplets, we consider the kinematics of a negatively buoyant probe particle in an active fluid, both confined within a spherical domain. The active bath generates a fluctuating flow that pushes the particle with a velocity that is modeled as a colored stochastic noise, characterized by two parameters, the intensity and memory time of the active flow. When the particle departs a little from the bottom of the spherical domain, the configuration is well approximated by a particle in a two-dimensional harmonic trap subjected to the colored noise, in which case an analytical solution exists, which is the base for quantitative analysis. We numerically simulate the dynamics of the particle and use the planar, two-dimensional mean square displacement to recover the activity parameters of the bath. This approach yields satisfactory results as long as the particle remains relatively confined, that is, as long as the intensity of the colored noise remains low.

cond-mat.soft↗

arXiv2023

Accumulation and depletion of E. coli in surfaces mediated by curvature

Can topography be used to control bacteria accumulation? We address this question in the model system of smooth-swimming and run-and-tumble \textit{Escherichia coli} swimming near a sinusoidal surface, and show that the accumulation of bacteria is determined by the characteristic curvature of the surface. For low curvatures, cells swim along the surface due to steric alignment and are ejected from the surface when they reach the peak of the sinusoid. Increasing curvature enhances this effect and reduces the density of bacteria in the curved surface. However, for curvatures larger than $κ^*\approx 0.3 μm^{-1}$, bacteria become trapped in the valleys, where they can remain for long periods of time. Minimal simulations considering only steric interactions with the surface reproduce these results and give insights into the physical mechanisms defining the critical curvature, which is found to scale with the inverse of the bacterial length. We show that for curvatures larger than $κ^*$, the otherwise stable alignment with the wall becomes unstable while the stable orientation is now perpendicular to the wall, thus predicting accurately the onset of trapping at the valleys.

cond-mat.soft↗

arXiv2014

The effect of confinement on the deformation of microfluidic drops

We study the deformation of drops squeezed between the floor and ceiling of a microchannel and subjected to a hyperbolic flow. We observe that the maximum deformation of drops depends on both the drop size and the rate of strain of the external flow and can be described with power laws with exponents 2.59 +/- 0.28 and 0.91 +/- 0.05 respectively. We develop a theoretical model to describe the deformation of squeezed drops based on the Darcy approximation for shallow geometries and the use of complex potentials. The model describes the steady-state deformation of the drops as a function of a non-dimensional parameter Ca d^2, where Ca is the capillary number (proportional to the strain rate and the drop size) and d is a confinement parameter equal to the drop size divided by the channel height. For small deformations, the theoretical model predicts a linear relationship between the deformation of drops and this parameter, in good agreement with the experimental observations.

physics.flu-dyn↗

arXiv2011

Quantitative analysis of the dripping and jetting regimes in co-flowing capillary jets

We study a liquid jet that breaks up into drops in an external co-flowing liquid inside a confining microfluidic geometry. The jet breakup can occur right after the nozzle in a phenomenon named dripping or through the generation of a liquid jet that breaks up a long distance from the nozzle, which is called jetting. Traditionally, these two regimes have been considered to reflect the existence of two kinds of spatiotemporal instabilities of a fluid jet, the dripping regime corresponding to an absolutely unstable jet and the jetting regime to a convectively unstable jet. Here, we present quantitative measurements of the dripping and jetting regimes, both in an unforced and a forced state, and compare these measurements with recent theoretical studies of spatiotemporal instability of a confined liquid jet in a co-flowing liquid. In the unforced state, the frequency of oscillation and breakup of the liquid jet is measured and compared to the theoretical predictions. The dominant frequency of the jet oscillations as a function of the inner flow rate agrees qualitatively with the theoretical predictions in the jetting regime but not in the dripping regime. In the forced state, achieved with periodic laser heating, the dripping regime is found to be insensitive to the perturbation and the frequency of drop formation remains unaltered. The jetting regime, on the contrary, amplifies the externally imposed frequency, which translates in the formation of drops at the frequency imposed by the external forcing. In conclusion, the dripping and jetting regimes are found to exhibit the main features of absolutely and convectively unstable flows respectively, but the frequency selection in the dripping regime is not ruled by the absolute frequency predicted by the stability analysis.

physics.flu-dyn↗