How to steer active colloids up a vertical wall.

An important challenge in active matter lies in harnessing useful global work from entities that produce work locally, e.g., via self-propulsion. We investigate here the active matter version of a classical capillary rise effect, by considering a non-phase separated sediment of self-propelled Janus colloids in contact with a vertical wall. We provide experimental evidence of an unexpected and dynamic adsorption layer at the wall. Additionally, we develop a complementary numerical model that recapitulates the experimental observations. We show that an adhesive and aligning wall enhances the pre-existing polarity heterogeneity within the bulk, enabling polar active particles to climb up a wall against gravity, effectively powering a global flux. Such steady-state flux has no equivalent in a passive wetting layer.

Continuous Manipulation and Characterization of Colloidal Beads and Liposomes via Diffusiophoresis in Single- and Double-Junction Microchannels.

We reveal a physical mechanism that enables the preconcentration, sorting, and characterization of charged polystyrene nanobeads and liposomes dispersed in a continuous flow within a straight micron-sized channel. Initially, a single Ψ-junction microfluidic chip is used to generate a steady-state salt concentration gradient in the direction perpendicular to the flow. As a result, fluorescent nanobeads dispersed in the electrolyte solutions accumulate into symmetric regions of the channel, appearing as two distinct symmetric stripes when the channel is observed from the top via epi-fluorescence microscopy. Depending on the electrolyte flow configuration and, thus, the direction of the salt concentration gradient field, the fluorescent stripes get closer to or apart from each other as the distance from the inlet increases. Our numerical and experimental analysis shows that although nanoparticle diffusiophoresis and hydrodynamic effects are involved in the accumulation process, diffusio-osmosis along the top and bottom channel walls plays a crucial role in the observed particles dynamics. In addition, we developed a proof-of-concept double Ψ-junction microfluidic device that exploits this accumulation mechanism for the size-based separation and size detection of nanobeads as well as for the measurement of zeta potential and charged lipid composition of liposomes under continuous flow settings. This device is also used to investigate the effect of fluid-like or gel-like states of the lipid membranes on the liposome diffusiophoretic response. The proposed strategy for solute-driven manipulation and characterization of colloids has great potential for microfluidic bioanalytical testing applications, including bioparticle preconcentration, sorting, sensing, and analysis.

Enhanced Accumulation of Colloidal Particles in Microgrooved Channels via Diffusiophoresis and Steady-State Electrolyte Flows.

The delivery of colloidal particles in dead-end microstructures is very challenging, since these geometries do not allow net flows of particle-laden fluids; meanwhile, diffusive transport is slow and inefficient. Recently, we introduced a novel particle manipulation strategy, based on diffusiophoresis, whereby the salt concentration gradient between parallel electrolyte streams in a microgrooved channel induces the rapid (i.e., within minutes) and reversible accumulation, retention, and removal of colloidal particles in the microgrooves. In this study, we investigated the effects of salt contrast and groove depth on the accumulation process in silicon microgrooves and determined the experimental conditions that lead to a particle concentration peak of more than four times the concentration in the channel bulk. Also, we achieved an average particle concentration in the grooves of more than twice the concentration in the flowing streams and almost 2 orders of magnitude larger than the average concentration in the grooves in the absence of a salt concentration gradient. Analytical sufficient and necessary conditions for particle accumulation are also derived. Finally, we successfully tested the accumulation process in polydimethylsiloxane microgrooved channels, as they are less expensive to fabricate than silicon microgrooved substrates. The controlled and enhanced accumulation of colloidal particles in dead-end structures by solute concentration gradients has potential applications in soft matter and living systems, such as drug delivery, synthetic biology, and on-chip diagnostics.

Aging or DEAD: Origin of the non-monotonic response to weak self-propulsion in active glasses.

Among amorphous states, glass is defined by relaxation times longer than the observation time. This nonergodic nature makes the understanding of glassy systems an involved topic, with complex aging effects or responses to further out-of-equilibrium external drivings. In this respect, active glasses made of self-propelled particles have recently emerged as a stimulating systems, which broadens and challenges our current understanding of glasses by considering novel internal out-of-equilibrium degrees of freedom. In previous experimental studies we have shown that in the ergodicity broken phase, the dynamics of dense passive particles first slows down as particles are made slightly active, before speeding up at larger activity. Here, we show that this nonmonotonic behavior also emerges in simulations of soft active Brownian particles and explore its cause. We refute that the deadlock by emergence of active directionality model we proposed earlier describes our data. However, we demonstrate that the nonmonotonic response is due to activity enhanced aging and thus confirm the link with ergodicity breaking. Beyond self-propelled systems, our results suggest that aging in active glasses is not fully understood.

Two-dimensional numerical model of Marangoni surfers: From single swimmer to crystallization.

We numerically study the dynamics of an ensemble of Marangoni surfers in a two-dimensional and unconfined space. The swimmers are modeled as Gaussian sources of surfactant generating surface tension gradients and are shown to follow the Marangoni flow filtered at their spatial scale in the lubrication regime, an unstable situation leading to spontaneous motion as soon as the Marangoni effect is intense enough. As the system is fully unconstrained, it is possible to study the various dynamical regimes from single swimmer, two-body interaction, to the many-particles case characterized by an efficient particle dispersion. We show that, although the present model is very simple, it reproduces the experimentally observed transition between a regime of dispersion by random agitation when the number of swimmers is moderate to the regime of crystallization with imperfect hexagonal lattice at high density.

Apparent Non-Newtonian Behavior of Ionic Liquids.

A significant viscosity variation with the shear rate has been observed for several ionic liquids in rheometry experiments above a critical shear rate. Depending on the liquid and the rheological conditions, both viscosity increase and decrease have been reported. So far, these variations have been interpreted as a signature of a non-Newtonian behavior. However, the measured critical shear rates are orders of magnitude below the ones predicted by numerical simulations. In this work, we perform new rheometry experiments with both ionic liquids and Newtonian liquids to elucidate this discrepancy. For these two types of liquids, both a viscosity decrease and increase have been measured depending on the geometry of the rheometer and the zero-shear viscosity of the liquid. We interpret the viscosity decrease as resulting from viscous heating, since the viscosity of the investigated liquids is also highly temperature-dependent, and the viscosity increase as resulting from the development of instabilities at high shear rates.

Reversible Trapping of Colloids in Microgrooved Channels via Diffusiophoresis under Steady-State Solute Gradients.

The controlled transport of colloids in dead-end structures is a key capability that can enable a wide range of applications, such as biochemical analysis, drug delivery, and underground oil recovery. This Letter presents a new trapping mechanism that allows the fast (i.e., within a few minutes) and reversible accumulation of submicron particles within dead-end microgrooves by means of parallel streams with different salinity level. For the first time, particle focusing in dead-end structures is achieved under steady-state gradients. Confocal microscopy analysis and numerical investigations show that the particles are trapped at a flow recirculation region within the grooves due to a combination of diffusiophoresis transport and hydrodynamic effects. Counterintuitively, the particle velocity at the focusing point is not vanishing and, hence, the particles are continuously transported in and out of the focusing point. The accumulation process is also reversible and one can cyclically trap and release the colloids by controlling the salt concentration of the streams via a flow switching valve.

Self-propulsion of symmetric chemically active particles: Point-source model and experiments on camphor disks.

Solid undeformable particles surrounded by a liquid medium or interface may propel themselves by altering their local environment. Such nonmechanical swimming is at work in autophoretic swimmers, whose self-generated field gradient induces a slip velocity on their surface, and in interfacial swimmers, which exploit unbalance in surface tension. In both classes of systems, swimmers with intrinsic asymmetry have received the most attention but self-propulsion is also possible for particles that are perfectly isotropic. The underlying symmetry-breaking instability has been established theoretically for autophoretic systems but has yet to be observed experimentally for solid particles. For interfacial swimmers, several experimental works point to such a mechanism, but its understanding has remained incomplete. The goal of this work is to fill this gap. Building on an earlier proposal, we first develop a point-source model that may be applied generically to interfacial or phoretic swimmers. Using this approximate but unifying picture, we show that they operate in very different regimes and obtain analytical predictions for the propulsion velocity and its dependence on swimmer size and asymmetry. Next, we present experiments on interfacial camphor disks showing that they indeed self-propel in an advection-dominated regime where intrinsic asymmetry is irrelevant and that the swimming velocity increases sublinearly with size. Finally, we discuss the merits and limitations of the point-source model in light of the experiments and point out its broader relevance.

Active Glass: Ergodicity Breaking Dramatically Affects Response to Self-Propulsion.

We study experimentally the response of a dense sediment of Brownian particles to self-propulsion. We observe that the ergodic supercooled liquid relaxation is monotonically enhanced by activity. By contrast the nonergodic glass shows an order of magnitude slowdown at low activities with respect to the passive case, followed by fluidization at higher activities. Our results contrast with theoretical predictions of the ergodic approach to glass transition, summing up to a shift of the glass line. We propose that nonmonotonicity is due to competing effects of activity: (i) extra energy that helps breaking cages; (ii) directionality that hinders cage exploration. We call it "deadlock from the emergence of active directionality." It suggests further theoretical works should include thermal motion.

Nonmonotonic behavior in dense assemblies of active colloids.

We study experimentally a sediment of self-propelled Brownian particles with densities ranging from dilute to ergodic supercooled to nonergodic glass to nonergodic polycrystal. In a companion paper, we observe a nonmonotonic response to activity of relaxation of the nonergodic glass state: a dramatic slowdown when particles become weakly self-propelled, followed by a speedup at higher activities. Here we map ergodic supercooled states to standard passive glassy physics, provided a monotonic shift of the glass packing fraction and the replacement of the ambient temperature by the effective temperature. However, we show that this mapping fails beyond glass transition. This failure is responsible for the nonmonotonic response. Furthermore, we generalize our finding by examining the dynamical response of another class of nonergodic systems: polycrystals. We observe the same nonmonotonic response to activity. To explain this phenomenon, we measure the size of domains where particles move in the same direction. This size also shows a nonmonotonic response, with small lengths corresponding to slow relaxation. This suggests that the failure of the mapping of nonergodic active states to a passive situation is general and is linked to anisotropic relaxation mechanisms specific to active matter.

Velocity Condensation for Magnetotactic Bacteria.

Magnetotactic swimmers tend to align along magnetic field lines against stochastic reorientations. We show that the swimming strategy, e.g., active Brownian motion versus run-and-tumble dynamics, strongly affects the orientation statistics. The latter can exhibit a velocity condensation whereby the alignment probability density diverges. As a consequence, we find that the swimming strategy affects the nature of the phase transition to collective motion, indicating that Lévy run-and-tumble walks can outperform active Brownian processes as strategies to trigger collective behavior.

Water Drop Evaporation on Mushroom-like Superhydrophobic Surfaces: Temperature Effects.

We report on experiments of drop evaporation on heated superhydrophobic surfaces decorated with micrometer-sized mushroom-like pillars. We analyze the influence of two parameters on the evaporation dynamics: the solid-liquid fraction and the substrate temperature, ranging between 30 and 80 °C. In the different configurations investigated, the drop evaporation appears to be controlled by the contact line dynamics (pinned or moving). The experimental results show that (i) in the pinned regime, the depinning angles increase with decreasing contact fraction and the substrate heating promotes the contact line depinning and (ii) in the moving regime, the droplet motion is described by periodic stick-slip events and contact-angle oscillations. These features are highly smoothed at the highest temperatures, with two possible mechanisms suggested to explain such a behavior, a reduction in the elasticity of the triple line and a decrease in the depinning energy barriers. For all surfaces, the observed remarkable stability of the "fakir" state to the temperature is attributed to the re-entrant micropillar curvature that prevents surface imbibition.

[Optimal permeability of aquaporins: a question of shape?]. / Perméabilité optimale des aquaporines - Une histoire de forme ?

Aquaporins are transmembrane proteins, ubiquitous in the human body. Inserted into the cell membranes, they play an important role in filtration, absorption and secretion of fluids. However, the excellent compromise between selectivity and permeability of aquaporins remains elusive. In this review, we focus on the hourglass shape of aquaporins, and we investigate its influence on water permeability, using numerical calculations and a simple theoretical model. We show that there is an optimum opening angle that maximizes the hydrodynamic permeability, and whose value is close to the angles observed in aquaporins.

How a "pinch of salt" can tune chaotic mixing of colloidal suspensions.

Efficient mixing of colloids, particles or molecules is a central issue in many processes. It results from the complex interplay between flow deformations and molecular diffusion, which is generally assumed to control the homogenization processes. In this work we demonstrate on the contrary that despite fixed flow and self-diffusion conditions, the chaotic mixing of colloidal suspensions can be either boosted or inhibited by the sole addition of a trace amount of salt as a co-mixing species. Indeed, this shows that local saline gradients can trigger a chemically driven transport phenomenon, diffusiophoresis, which controls the rate and direction of molecular transport far more efficiently than the usual Brownian diffusion. A simple model combining the elementary ingredients of chaotic mixing with diffusiophoretic transport of the colloids allows rationalization of our observations and highlights how small-scale out-of-equilibrium transport bridges to mixing at much larger scales in a very effective way. Considering chaotic mixing as a prototypal building block for turbulent mixing suggests that these phenomena, occurring whenever the chemical environment is inhomogeneous, might bring interesting perspectives from micro-systems to large-scale situations, with examples ranging from ecosystems to industrial contexts.

Effect of surface elasticity on the rheology of nanometric liquids.

The rheological properties of liquids confined to nanometer scales are important in many physical situations. In this Letter, we demonstrate that the long-range elastic deformation of the confining surfaces must be taken into account when considering the rheology of nanometric liquids. In the case of a squeeze-flow geometry, we show that below a critical distance D(c), the liquid is clamped by its viscosity and its intrinsic properties cannot be disentangled from the global system response. Using nanorheology experiments, we demonstrate that picometer elastic deflections of the rigid confining surfaces dominate the overall mechanical response of nanometric liquids confined between solid walls.

Capacitive detection of buried interfaces by a dynamic surface force apparatus.

We present here a new type of distance sensor mounted on a Surface Force Apparatus (SFA), able to detect the position of a buried interface and therefore the thickness of a thin solid or soft matter film coating the SFA surface(s). This sensor relies on the capacitance created by the two metallized surfaces of the SFA. An harmonic oscillation of these polarized surfaces creates a pico- to femto-amps current indicating their relative position. One of the specificities of this sensor is the relatively weak polarization used for the measurements, minimizing the electrical forces and their impact on other interactions, hydrodynamical and mechanical forces measured by the SFA. This original and simple design is of high interest for studying the viscoelastic properties of thin films, to detect adsorbed liquid layers or slippage at liquid-solid interfaces, or even to study complex fluids such as ionic liquids under polarization. We demonstrate the use of this sensor to study the flow boundary condition of silicon oil on a metal surface, and the elastic modulus of a thin elastomer layer.

Optimizing water permeability through the hourglass shape of aquaporins.

The ubiquitous aquaporin channels are able to conduct water across cell membranes, combining the seemingly antagonist functions of a very high selectivity with a remarkable permeability. Whereas molecular details are obvious keys to perform these tasks, the overall efficiency of transport in such nanopores is also strongly limited by viscous dissipation arising at the connection between the nanoconstriction and the nearby bulk reservoirs. In this contribution, we focus on these so-called entrance effects and specifically examine whether the characteristic hourglass shape of aquaporins may arise from a geometrical optimum for such hydrodynamic dissipation. Using a combination of finite-element calculations and analytical modeling, we show that conical entrances with suitable opening angle can indeed provide a large increase of the overall channel permeability. Moreover, the optimal opening angles that maximize the permeability are found to compare well with the angles measured in a large variety of aquaporins. This suggests that the hourglass shape of aquaporins could be the result of a natural selection process toward optimal hydrodynamic transport. Finally, in a biomimetic perspective, these results provide guidelines to design artificial nanopores with optimal performances.

Hydrodynamic interaction between a spherical particle and an elastic surface: a gentle probe for soft thin Films.

We study the hydrodynamic interaction between a sphere and an elastic surface at a nanoscale with a dynamic surface force apparatus. We show that the interplay between viscous forces and elastic deformations leads to very rich scaling properties of the force response, providing a unique signature of the surface elastic behavior. These properties are illustrated on three different examples: a thick elastomer, a thin elastomer film, and a layer of micrometric bubbles. We show that this fluid probing allows one to measure the Young's modulus of surfaces and soft thin layers at distance, without any direct solid-solid contact.

Sedimentation and effective temperature of active colloidal suspensions.

In this Letter, we investigate experimentally the nonequilibrium steady state of an active colloidal suspension under gravity field. The active particles are made of chemically powered colloids, showing self propulsion in the presence of an added fuel, here hydrogen peroxide. The active suspension is studied in a dedicated microfluidic device, made of permeable gel microstructures. Both the microdynamics of individual colloids and the global stationary state of the suspension under gravity are measured with optical microscopy. This yields a direct measurement of the effective temperature of the active system as a function of the particle activity, on the basis of the fluctuation-dissipation relationship. Our work is a first step in the experimental exploration of the out-of-equilibrium properties of active colloidal systems.

Colloidal motility and pattern formation under rectified diffusiophoresis.

In this Letter, we characterize experimentally the diffusiophoretic motion of colloids and lambda-DNA toward higher concentration of solutes, using microfluidic technology to build spatially and temporally controlled concentration gradients. We then demonstrate that segregation and spatial patterning of the particles can be achieved from temporal variations of the solute concentration profile. This segregation takes the form of a strong trapping potential, stemming from an osmotically induced rectification mechanism of the solute time-dependent variations. Depending on the spatial and temporal symmetry of the solute signal, localization patterns with various shapes can be achieved. These results highlight the role of solute contrasts in out-of-equilibrium processes occurring in soft matter.