An electrokinetic route to giant augmentation in load bearing capacity of compliant microfluidic channels

The performances of lubricated systems widely used in natural, biological, and artificial settings are traditionally dictated by their load bearing capacities. Here we unveil that, by exploiting a unique coupling between interfacial electro-mechanics, hydrodynamics and substrate compliance, it is plausible to realize a massive augmentation in the load bearing capacities ofcompliant microfluidic channels. Our analysis demonstrates that the interplay between wettability and charge modulation in association with the solution chemistry and surface compliance results in this remarkable phenomenon. These results are likely to open up novel design paradigms of augmenting the load bearing capacities of miniaturized bio-mimetic units through the realization of a symmetry breaking phenomenon triggered by asymmetries in electromechanical and hydrodynamic transport over interfacial scales.Comment: 29 pages, 7 figure

Convective dissolution of CO2_2 in 2D and 3D porous media: the impact of hydrodynamic dispersion

Convective dissolution is the process by which CO$_2$ injected in deep geological formations dissolves into the aqueous phase, which allows storing it perennially by gravity. The process results from buoyancy-coupled Darcy flow and solute transport. Proper theoretical modeling of the process should consider in the transport equation a diffusive term accounting for hydrodynamics (or, mechanical) dispersion, with an effective diffusion coefficient that is proportional to the local interstitial velocity. A few two-dimensional (2D) numerical studies, and three-dimensional (3D) experimental investigations, have investigated the impact of hydrodynamic dispersion on convection dynamics, with contradictory conclusions. Here, we investigate systematically the impact of the dispersion strength $S$ (relative to molecular diffusion), and of the anisotropy $\alpha$ of its tensor, on convective dissolution in 2D and 3D geometries. We use a new numerical model and analyze the solute fingers' number density (FND), penetration depth and maximum velocity; the onset time of convection; the dissolution flux in the quasi-constant flux regime; the mean concentration of the dissolved CO2; and the scalar dissipation rate. The efficiency of convective dissolution over long times is observed to be mostly controlled by the onset time of convection. For most natural porous media ($\alpha = 0.1$), the onset time is found to increase as a function of $S$, in agreement with previous experimental findings and in stark contrast to previous numerical findings. However, if $\alpha$ is sufficiently large this behavior is reversed. Furthermore, results in 3D are fully consistent with the 2D results on all accounts, except that in 3D the onset time is slightly smaller, the dissolution flux in the quasi-constant flux regime is slightly larger, and the dependence of the FND on the dispersion parameters is impacted by $Ra$.Comment: 30 pages, 18 figure

Phytoplankton tune local pH to actively modulate circadian swimming behavior

Diel vertical migration (DVM), the diurnal exodus of motile phytoplankton between the light- and nutrient-rich aquatic regions, is governed by endogenous biological clocks. Many species exhibit irregular DVM patterns wherein out-of-phase gravitactic swimming–relative to that expected due to the endogenous rhythm–is observed. How cells achieve and control this irregular swimming behavior remains poorly understood. Combining local environmental monitoring with behavioral and physiological analyses of motile bloom-forming Heterosigma akaswhiwo cells, we report that phytoplankton species modulate their DVM pattern by progressively tuning local pH, yielding physiologically equivalent yet behaviorally distinct gravitactic sub-populations which remain separated vertically within a visibly homogeneous cell distribution. Individual and population-scale tracking of the isolated top and bottom sub-populations revealed similar gravitactic (swimming speed and stability) and physiological traits (growth rate and maximum photosynthetic yield), suggesting that the sub-populations emerge due to mutual co-existence. Exposing the top (bottom) sub-population to the spent media of the bottom (top) counterpart recreates the emergent vertical distribution, while no such phenomenon was observed when the sub-populations were exposed to their own spent media. A model of swimming mechanics based on the quantitative analysis of cell morphologies confirms that the emergent sub-populations represent distinct swimming stabilities, resulting from morphological transformations after the cells are exposed to the spent media. Together with the corresponding night-time dataset, we present an integrated picture of the circadian swimming, wherein active chemo-regulation of the local environment underpins motility variations for potential ecological advantages via intraspecific division of labor over the day-night cycle. This chemo-regulated migratory trait offers mechanistic insights into the irregular diel migration, relevant particularly for modelling phytoplankton transport, fitness and adaptation as globally, ocean waters see a persistent drop in the mean pH.

Self-regulation of phenotypic noise synchronizes emergent organization and active transport in confluent microbial environments

The variation associated with different observable characteristics—phenotypes—at the cellular scale underpins homeostasis and the fitness of living systems. However, if and how these noisy phenotypic traits shape properties at the population level remains poorly understood. Here we report that phenotypic noise self-regulates with growth and coordinates collective structural organization, the kinetics of topological defects and the emergence of active transport around confluent colonies. We do this by cataloguing key phenotypic traits in bacteria growing under diverse conditions. Our results reveal a statistically precise critical time for the transition from a monolayer biofilm to a multilayer biofilm, despite the strong noise in the cell geometry and the colony area at the onset of the transition. This reveals a mitigation mechanism between the noise in the cell geometry and the growth rate that dictates the narrow critical time window. By uncovering how rectification of phenotypic noise homogenizes correlated collective properties across colonies, our work points at an emergent strategy that confluent systems employ to tune active transport, buffering inherent heterogeneities associated with natural cellular environment settings