Cells and tissues have the remarkable ability to actively generate the forces required to change theirshape. This active mechanical behavior is largely mediated by the actin cytoskeleton, a crosslinkednetwork of actin filaments that is contracted by myosin motors. Experiments and active gel theorieshave established that the length scale over which gel contraction occurs is governed by a balancebetween molecular motor activity and crosslink density. By contrast, the dynamics that govern thecontractile activity of the cytoskeleton remain poorly understood. Here we investigate the microscopicdynamics of reconstituted actin–myosin networks using simultaneous real-space video microscopy andFourier-space dynamic light scattering. Light scattering reveals different regimes of microscopicdynamics as a function of sample age. We uncover two dynamical precursors that precede macroscopicgel contraction. One is characterized by a progressive acceleration of stress-induced rearrangements,while the other consists of sudden, heterogeneous rearrangements. Intriguingly, our findings suggest aqualitative analogy between self-driven rupture and collapse of active gels and the delayed rupture ofpassive gels observed in earlier studies of colloidal gels under external loads.

The peculiar linear temperature-dependent swelling of core-shell microgels has been conjectured to be linked to the core-shell architecture combining materials of different transition temperatures. Here the structure of pNIPMAM-core and pNNPAM-shell microgels in water is studied as a function of temperature using small-angle neutron scattering with selective deuteration. Photon correlation spectroscopy is used to scrutinize the swelling behaviour of the colloidal particles and reveals linear swelling. Moreover, these experiments are also employed to check the influence of deuteration on swelling. Using a form free multi-shell reverse Monte Carlo approach, the small-angle scattering data are converted into radial monomer density profiles. The comparison of ‘core-only’ particles consisting of identical cores to fully hydrogenated core-shell microgels, and finally to H core/D shell architectures unambiguously shows that core and shell monomers display gradient profiles with strong interpenetration, leading to cores embedded in shells which are bigger than their isolated ‘core only’ precursor particles. This surprising result is further generalized to different core cross linker contents, for temperature ranges encompassing both transitions. Our analysis demonstrates that the internal structure of pNIPMAM-core and pNNPAM-shell microgels is heterogeneous and strongly interpenetrated, presumably allowing only progressive core swelling at temperatures intermediate to both transition temperatures, thus promoting linear swelling behaviour.

We have investigated the wetting and surface diffusion of mesoporous colloidal silica particles at the water surface and the adsorption of cationic cetyltrimethylammonium (CTA+) surfactant on these particles. Porous silica colloids diffuse at the surface of water and in the volume, interacting with cationic surfactants that can adsorb inside the pores of the particles. We observed that surfactant adsorption on mesoporous silica depends dramatically not only on the particle pore size but also on specific counterion effects. We measured striking differences both on a macroscopic property of the interface, i.e., surface tension, and also at a single particle level by evaluating the translational diffusion of partially wetted particles at the fluid interface. We varied the pore size from 2 to 7 nm and explored the effects of ions possessing different hydration number and kosmotropic/chaotropic character. At concentrations lower than the critical micellar concentration, we evidence that cationic surfactants adsorb on silica as surface micelles and surfactant adsorption inside the pores occurs only if the pore diameter is larger than the size of surface micelles. With a view to understand the surprising different adsorption behavior of CTA+OH– and CTA+Br– on porous silica particles, we investigated the effect of counterions on the surfactant adsorption on porous silica colloids by tuning the pH and the counterion properties.

In previous work [Opt. Lett. 44, 2827 (2019)], we presented a method based on digital holography and orthogonal matching pursuit, which is able to determine the 3D positions of small objects moving within a larger motionless object. Indeed, if the scattering density is sparse in direct 3D space, compressive sensing algorithms can be used. The method was validated by imaging red blood cell trajectories in the trunk vascular system of a zebrafish (Danio rerio) larva. We give here further details on the reconstruction technique and present a more robust version of the algorithm based on multiple illuminations.

Soft adhesion to a horizontal flat substrate of micron-sized colloids coated by acontrolled molar fraction f of PLL-g-PNIPAM is tuned and triggered by controllingthe temperature of the sample. This mechanism is possible becausePNIPAM is a thermo-responsive polymer, which undergoes a phase transitionat a LCST (lower critical solution temperature) Tc = 32 +- 1°C. In order tocapture the very final events before the immobilization of colloids the T-rampprotocol is designed: the particles suspension is injected in cell at room temperature,the temperature is increased at a constant rate up to 38°C> Tc, andkept constant until the end of the acquisition. Attraction between beads andthe flat substrate is thereby triggered when crossing the critical temperatureTc. Ascending and descending ramp experiments are performed in order to accessboth adsorption and desorption kinetics. 3-D motion of beads is real-timetracked using slightly defocused microscopy in parallel illumination. We usetrack records to have access to pre-adsorption dffusion coeffcients and to characterizeadsorption and desorption dynamics. Present results corroborate thatadsorption is controlled by PNIPAM doping, and indeed dominated by rollingand memory (aging) effect on the contact domain. On the contrary, the desorption kinetics is independent of doping. Moreover, the sharpness (in T) ofthe PNIPAM transition is quantified in a set of experiments at different rampspeeds. We show that the transition smearing can reach up to +-1°C for higherPNIPAM coverage, while it is not detectable for the lowest ones. The combinationof all these observations paves the way to practical applications. Asan example, we will discuss a soft adhesion-based method to sort and separatecolloids in microfluidic channels.

Secondary flows are ubiquitous in channel flows, where small velocity components perpendicular to the main velocity appear due to the complexity of the channel geometry and/or that of the flow itself such as from inertial or non-Newtonian effects, etc. We investigate here the inertialess secondary flow of viscoelastic fluids in curved microchan-nels of rectangular cross-section and constant but alternating curvature: the so-called "serpentine channel" geometry. Numerical calculations (Poole et al, 2013) have shown that in this geometry, in the absence of elastic instabilities, a steady secondary flow develops that takes the shape of two counter-rotating vortices in the plane of the channel cross-section. We present the first experimental visualization evidence and characterization of these steady secondary flows, using a complementarity of µPIV in the plane of the channel, and L. Ducloué · L. Casanellas · A. Lindner confocal visualisation of dye-stream transport in the cross-sectional plane. We show that the measured streamlines and the relative velocity magnitude of the secondary flows are in qualitative agreement with the numerical results. In addition to our techniques being broadly applicable to the character-isation of three-dimensional flow structures in microchan-nels, our results are important for understanding the onset of instability in serpentine viscoelastic flows.