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Microbes thrive in diverse porous environments -- from soil and riverbeds to human lungs and cancer tissues -- spanning multiple scales and conditions. Short- to long-term fluctuations in local factors induce spatio-temporal heterogeneities, often leading to physiologically stressful settings. How microbes respond and adapt to such biophysical constraints is an active field of research where considerable insight has been gained over the last decade and a half. With a focus on bacteria, here we review recent advances in microbial self-organization and dispersal in inorganic and organic porous settings, highlighting the role of active interactions and feedback which mediate their survival and fitness. We conclude by discussing open questions and opportunities for leveraging integrative cross-disciplinary approaches to advance our understanding of the biophysical strategies that microbes employ -- at both species and community scales -- to make porous settings habitable. Active and responsive behaviour is key to microbial survival in porous environments, with far-reaching ramifications for developing strategies to mitigate anthropogenic impacts, innovate subsurface storage solutions, and predict future ecological scenarios imposed by current climatic changes., Comment: Literature review, 41 pages, 6 figures

Accurate tracking of cellular and subcellular structures, along with their dynamics, plays a pivotal role in understanding the underlying mechanisms of biological systems. This paper presents a novel approach, ProGroTrack, that combines the You Only Look Once (YOLO) and ByteTrack algorithms within the detection-based tracking (DBT) framework to track intracellular protein nanostructures. Focusing on iPAK4 protein fibers as a representative case study, we conducted a comprehensive evaluation of YOLOv5 and YOLOv8 models, revealing the superior performance of YOLOv5 on our dataset. Notably, YOLOv5x achieved an impressive mAP50 of 0.839 and F-score of 0.819. To further optimize detection capabilities, we incorporated semi-supervised learning for model improvement, resulting in enhanced performances in all metrics. Subsequently, we successfully applied our approach to track the growth behavior of iPAK4 protein fibers, revealing their two distinct growth phases consistent with a previously reported kinetic model. This research showcases the promising potential of our approach, extending beyond iPAK4 fibers. It also offers a significant advancement in precise tracking of dynamic processes in live cells, and fostering new avenues for biomedical research.

We perform numerical and experimental studies on the melting process of thin films initiated by a small hole. The presence of a non-trivial capillary surface, namely the liquid/air interface, leads to a few counter-intuitive results: (1) The melting point is elevated if the film surface is partially wettable, even with a small contact angle. (2) For a film that is finite in size, melting may prefer to start from the outer boundary, rather than a hole inside. (3) More complex melting scenario may arise, including morphology transitions, and the "de facto" melting point being a range instead of a single value. These are verified by experiments on melting alkane films between silica and air. This work continues a series of investigations on the capillary aspects on melting. Both our model and analysis approach can be easily generalized to other systems., Comment: 7 pages, 7 figures

Micro-swimmers can serve as cargo carriers that move deep inside complex flow networks. When a school collectively entrains the surrounding fluid, their transport capacity can be enhanced. This effect is quantified with good agreement between experiments with self-propelled droplets and a confined Brinkman squirmer model. The volume of liquid entrained can be much larger than the droplet itself, amplifying the effective cargo capacity over an order of magnitude, even for dilute schools. Hence, biological and engineered swimmers can efficiently transport materials into confined environments., Comment: 15 pages, 3 videos. PRL, in press. https://journals.aps.org/prl/accepted/92073Y5eQ3214f79977928f65ce99424e72a173bb

Biological microswimmers are known to navigate upstream of an external flow (positive rheotaxis) in trajectories ranging from linear, spiral to oscillatory. Such rheotaxis stems from the interplay between the motion and complex shapes of the microswimmers, e.g. the chirality of the rotating flagella, the shear flow characteristics, and the hydrodynamic interaction with a confining surface. Here, we show that an isotropic, active droplet microswimmer exhibits a unique oscillatory rheotaxis in a microchannel despite its simple spherical geometry. The swimming velocity, orientation, and the chemical wake of the active droplet undergo periodic variations between the confining walls during the oscillatory navigation. Using a hydrodynamic model and concepts of dynamical systems, we demonstrate that the oscillatory rheotaxis of the active droplet emerges primarily from the interplay between the hydrodynamic interaction of the finite-sized microswimmer with all the microchannel walls, and the shear flow characteristics. Such oscillatory rheotactic behavior is different from the directed motion near a planar wall observed previously for artificial microswimmers in shear flows. Our results provide a realistic understanding of the behaviour of active particles in confined microflows, as will be encountered in majority of the applications like targeted drug delivery.

We study the detention statistics of self-propelling droplet microswimmers attaching to microfluidic pillars. These droplets show negative autochemotaxis: they shed a persistent repulsive trail of spent fuel that biases them to detach from pillars of a certain size after orbiting them just once. We have designed a microfluidic assay recording swimmers in pillar arrays of varying diameter, derived detention statistics via digital image analysis and interpreted these statistics via the Langevin dynamics of an active Brownian particle (ABP) model. By comparing data from orbits with and without residual chemical field, we can independently estimate quantities like hydrodynamic and chemorepulsive torques, chemical coupling constants and diffusion coefficients, as well as their dependence on boundary conditions like wall curvature.

It is commonly considered that the frustration between the curvature energy and the chain stretching energy plays an important role in the formation of lyotropic liquid crystals in bicontinuous cubic phases. Theoretic and numeric calculations were performed for two extreme cases: Parallel surfaces eliminate the variance of the chain length; constant mean curvature surfaces eliminate the variance of the mean curvature. We have implemented a model with Brakke's Surface Evolver which allows a competition between the two variances. The result shows a compromise of the two limiting geometries. With data from real systems, we are able to recover the G--D--P phase sequence which was observed in experiments., Comment: 19 pages, 13 figures. An alternative version was submitted for peer review

Chemotaxis and auto-chemotaxis play an important role in many essential biological processes. We present a self-propelling artificial swimmer system which exhibits chemotaxis as well as negative auto-chemotaxis. Oil droplets in an aqueous surfactant solution are driven by interfacial Marangoni flows induced by micellar solubilization of the oil phase. We demonstrate that chemotaxis along micellar surfactant gradients can guide these swimmers through a microfluidic maze. Similarly, a depletion of empty micelles in the wake of a droplet swimmer causes negative autochemotaxis and thereby trail avoidance. We have studied autochemotaxis quantitatively in a microfluidic device of bifurcating channels: Branch choices of consecutive swimmers are anticorrelated, an effect decaying over time due to trail dispersion. We have modeled this process by a simple one-dimensional diffusion process and stochastic Langevin dynamics. Our results are consistent with a linear surfactant gradient force and diffusion constants appropriate for micellar diffusion, and provide a measure of autochemotactic feedback strength versus stochastic forces.

Most studies on melting under confinement focus only on the solid and liquid melt phases. Despite of its ubiquity, contributions from the capillary interface (liquid / vapor interface) are often neglected. In this study the melting behavior of small cylindrical aggregates in vapor attached to planar surfaces is analyzed. For the assumed boundary conditions (cylindrical solid with a non wetting top plane and a wettable side wall) solid and the liquid phases can coexist within a certain temperature range. Due to capillary instability, the liquid phase can form either an axisymmetric rouloid morphology or, above a certain threshold liquid volume fraction, a bulge coexisting with a rouloid-like section. The corresponding melting points are different. The analysis explicitly describes the behavior of a real system of small aggregates of long chain alkanes on planar substrates. It also gives qualitative insights into the melting behavior of small aggregates with anisotropic wetting behaviors in general. It reveals in particular how melting points and melting pathways depend on the energetic respectively morphological pathways leading to complete melting., Comment: 12 pages, 7 figures. J. Phys. Chem. C, accepted

Motivated by unexpected morphologies of the emerging liquid phase (channels, bulges, droplets) at the edge of thin, melting alkane terraces, we propose a new heterogeneous nucleation pathway. The competition between bulk and interfacial energies and the boundary conditions determine the growth and shape of the liquid phase at the edge of the solid alkane terraces. Calculations and experiments reveal a "pre-critical" shape transition (channel-to-bulges) of the liquid before reaching its critical volume along a putative shape-conserving path. Bulk liquid emerges from the new shape, and depending on the degree of supersaturation, the new pathway may have two, one, or zero energy barriers. The findings are broadly relevant for many heterogeneous nucleation processes because the novel pathway is induced by common, widespread surface topologies (scratches, steps, etc.)., Comment: 5 figures, accepted for publication in Physical Review Letters (2012)