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Crosslinked and bundled actin filaments form networks that are essential for the mechanical properties of living cells. Reconstituted actin networks have been extensively studied not only as a model system for the cytoskeleton, but also to understand the interplay between microscopic structure and macroscopic viscoelastic properties of network-forming soft materials. These constitute a broad class of materials with countless applications in science and industry. So far, it has been widely assumed that reconstituted actin networks represent equilibrium structures. Here, we show that fully polymerized actin/fascin bundle networks exhibit surprising age-dependent changes in their viscoelastic properties and spontaneous dynamics, a feature strongly reminiscent of out-of-equilibrium, or glassy, soft materials. Using a combination of rheology, confocal microscopy and space-resolved dynamic light scattering, we demonstrate that actin networks build up stress during their formation and then slowly relax towards equilibrium owing to the unbinding dynamics of the crosslinking molecules. Actin networks are an excellent model system for studying the mechanical properties of the cell cytoskeleton. Using microscopic methods, actin bundle networks formed in the presence of the crosslinking protein fascin show age-dependent changes in their viscoelastic properties and spontaneous relaxation dynamics in a similar way to glassy, soft materials.

Understanding glass formation is a challenge, because the existence of a true glass state, distinct from liquid and solid, remains elusive: Glasses are liquids that have become too viscous to flow. An old idea, as yet unproven experimentally, is that the dynamics becomes sluggish as the glass transition approaches, because increasingly larger regions of the material have to move simultaneously to allow flow. We introduce new multipoint dynamical susceptibilities to estimate quantitatively the size of these regions and provide direct experimental evidence that the glass formation of molecular liquids and colloidal suspensions is accompanied by growing dynamic correlation length scales.

The evaporation of drops of colloidal suspensions plays an important role in numerous contexts, such as the production of powdered dairies, the synthesis of functional supraparticles, and virus and bacteria survival in aerosols or drops on surfaces. The presence of colloidal particles in the evaporating drop eventually leads to the formation of a dense shell that may undergo a shape instability. Previous works propose that, for drops evaporating very fast, the instability occurs when the particles form a rigid porous solid, constituted of permanently aggregated particles at random close packing. To date, however, no measurements could directly test this scenario and assess whether it also applies to drops drying at lower evaporation rates, severely limiting our understanding of this phenomenon and the possibility of harnessing it in applications. Here, we combine macroscopic imaging and space- and time-resolved measurements of the microscopic dynamics of colloidal nanoparticles in drying drops, measuring the evolution of the thickness of the shell and the spatial distribution and mobility of the nanoparticles. We find that, above a threshold evaporation rate, the drop undergoes successively two distinct shape instabilities. While the second instability is due to the permanent aggregation of nanoparticles, as hypothesized in previous works on fast-evaporating drops, we show that the first one results from a reversible glass transition of the shell, unreported so far. We rationalize our findings and discuss their implications in the framework of a unified state diagram for the drying of colloidal drops.

A wide variety of systems, including granular media, colloidal suspensions and molecular systems, exhibit non-equilibrium transitions from a fluid-like to a solid-like state, characterized solely by the sudden arrest of their dynamics. Crowding or jamming of the constituent particles traps them kinetically, precluding further exploration of the phase space 1 . The disordered fluid-like structure remains essentially unchanged at the transition. The jammed solid can be refluidized by thermalization, through temperature or vibration, or by an applied stress. The generality of the jamming transition led to the proposal 2 of a unifying description, based on a jamming phase diagram. It was further postulated that attractive interactions might have the same effect in jamming the system as a confining pressure, and thus could be incorporated into the generalized description. Here we study experimentally the fluid-to-solid transition of weakly attractive colloidal particles, which undergo markedly similar gelation behaviour with increasing concentration and decreasing thermalization or stress. Our results support the concept of a jamming phase diagram for attractive colloidal particles, providing a unifying link between the glass transition 3 , gelation 4 , 5 and aggregation 6 , 7 , 8 .

The key parameter controlling the glass transition of colloidal suspensions is \\(\\varphi\\), the fraction of the sample volume occupied by the particles. Unfortunately, changing \\(\\varphi\\) by varying an external parameter, \\textit{e.g.} temperature \\(T\\) as in molecular glass formers, is not possible, unless one uses thermosensitive colloidal particles, like the popular poly(N-isopropylacrylamide) (PNiPAM) microgels. These however have several drawbacks, including high deformability, osmotic deswelling and interpenetration, which complicate their use as a model system to study the colloidal glass transition. Here, we propose a new system consisting of a colloidal suspension of non-deformable spherical silica nanoparticles, in which PNiPAM hydrogel spheres of ~\\(100-200 \\mu m\\) size are suspended. These non-colloidal `mesogels' allow for controlling the sample volume effectively available to the silica nanoparticles and hence their \\(\\varphi\\), thanks to the \\(T\\)-induced change in mesogels volume. Using optical microscopy, we first show that the mesogels retain their ability to change size with \\(T\\) when suspended in Ludox suspensions, similarly as in water. We then show that their size is independent of the sample thermal history, such that a well-defined, reversible relationship between \\(T\\) and \\(\\varphi\\) may be established. Finally, we use space-resolved dynamic light scattering to demonstrate that, upon varying \\(T\\), our system exhibits a broad range of dynamical behaviors across the glass transition and beyond, comparable with those exhibited by a series of distinct silica nanoparticle suspensions of various \\(\\varphi\\).

A new type of hybrid material is prepared through sol-gel processing by the polycondensation of (MeO)3Si–R–Si(OMe)3 units containing a rigid organic anisotropic group R. The hybrid covalently bounded organic-inorganic gel shows an unexpected structural birefringence Δn (Δn = 2 × 10−3). This birefringence is induced by a strain field anisotropy during gel aging as revealed by simultaneous dynamic light scattering and birefringence measurements. To better address the role of the strain field, we use a free interface sol-gel/air to control strain anisotropy and to measure the gel optical axis and its birefringence. We find that the birefringence is associated to a long-range orientational order of organic moieties induced by the strain anisotropy during the gel aging.

To investigate the interplay between microscopic dynamics and macroscopic rheology in soft matter, we couple a stress-controlled-rheometer equipped with a Couette cell to a light scattering setup in the imaging geometry, which allows us to measure both the deformation field and the microscopic dynamics. To validate our setup, we test two model systems. For an elastic solid sample, we recover the expected deformation field within 1 micron. For a pure viscous fluid seeded with tracer particles, we measure the velocity profile and the dynamics of the tracers, both during shear and at rest. The velocity profile is acquired over a gap of 5 mm with a temporal and spatial resolution of 1 s and 100 microns, respectively. At rest, the tracer dynamics have the expected diffusive behavior. Under shear, the microscopic dynamics corrected for the average drift due to solid rotation scale with the local shear rate, demonstrating that our setup captures correctly the relative motion of the tracers due to the affine deformation.

We develop and thoroughly test a stress-controlled, parallel plates shear cell that can be coupled to an optical microscope or a small angle light scattering setup, for simultaneous investigation of the rheological properties and the microscopic structure of soft materials under an imposed shear stress. In order to minimize friction, the cell is based on an air bearing linear stage, the stress is applied through a contactless magnetic actuator, and the strain is measured through optical sensors. We discuss the contributions of inertia and of the small residual friction to the measured signal and demonstrate the performance of our device in both oscillating and step stress experiments on a variety of viscoelastic materials.

We introduce a temporal scheme for data sampling, based on a variable delay between two successive data acquisitions. The scheme is designed so as to reduce the average data flow rate, while still retaining the information on the data evolution on fast time scales. The practical implementation of the scheme is discussed and demonstrated in light scattering and microscopy experiments that probe the dynamics of colloidal suspensions using CMOS or CCD cameras as detectors.