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Invited speakers

Linda Hirst (UC Merced) (10:30-11:15)
Liquid Crystal Templated nano-assemblies
A current goal in nanotechnology focuses on the assembly of different nanoparticle types into 3D organized structures, although producing such materials by a simple, scalable method can be challenging. Recently our group has focused on liquid crystal/nanoparticle composites. In such materials nanoparticles of different types (semiconducting, metallic, magnetic etc) with defined surface properties are dispersed or assembled to produce multifunctional new composite materials. In this presentation I will review recent results from our group including the use of a liquid crystal host phase to generate micron-scale vesicle-like nanoparticle shells stabilized by ligand-ligand interactions. The constructs formed consist of a robust, thin spherical layer, composed of closely packed nanoparticles and stabilized by local interactions between mesogenic ligands. The assembly methods we describe can be extended to various nanoparticle types, where hollow shell formation is controlled by thermally sorting mesogen-functionalized nanoparticles in a liquid crystalline host material at the isotropic to nematic transition.
Pepijn Moerman (Utrecht University) (11:15-11:45)
Concentration gradient mediated interactions between active droplets
Autophoretic swimmers are subject to an effective interaction with each other that is due to overlap of the chemical gradients that surround them and drive their motion. It is believed to be similar to the interaction between micro-organisms, which also form and respond to solute gradients. An interesting feature of this interaction is that it involves the solvent, so it can be non-reciprocal. Even though many particles and micro-organisms interact this way, very few quantitative studies are available. The main obstacle is that this interaction is often coupled to self-propulsion making it impossible to study the two effects separately. We show that isotropic swimmers, geometrically symmetric active particles, only swim beyond a cutoff fuel concentration, providing a window of opportunity to study the solute mediated interaction without the complication of swimming. Using optical tweezers we quantified the force between isotropic swimmers due to this solute mediated interaction and show that the force scales with inter-particle distance as 1/r2, as is expected for a diffusion dominated process. We propose a functional form for the interaction, based on steady-state diffusion profiles, that accurately describes our data. This model is in principle applicable to all solute gradient induced interactions and can aid in understanding the behavior of micro-swimmers as well as more complicated biological systems.
Matthieu Wyart (EPFL Lausanne) (13.30-14.15)
On shear thickening
A consensus is emerging that discontinuous shear thickening (DST) in dense suspensions marks a transition from a flowing state where particles remain well separated by lubrication layers, to one dominated by frictional contacts. I will argue that reasonable assumptions about contact proliferation predict two distinct types of DST in the absence of inertia. The first occurs at densities above the jamming point of frictional particles; here the thickened state is completely jammed and (unless particles deform) cannot flow without inhomogeneity or fracture. The second regime shows strain-rate hysteresis and arises at somewhat lower densities where the thickened phase ows smoothly. DST is predicted to arise when finite-range repulsions defer contact formation until a characteristic stress level is exceeded. If time permits, I will discuss why a chaotic behavior appears in controlled stress experiments for systems displaying DST.
Anton Souslov (University of Leiden) (14.15-14.45)
Topological acoustics and synthetic gauge fields in active liquids and mechanical metamaterials
Topologically protected sound and light waves have been realized in materials in which time-reversal symmetry is broken via an external drive or field. Here, we take a different approach that relies on polar active liquids, i.e., fluids that flow spontaneously and without the need of an external drive. We control the spontaneous steady-state flow by confining the active liquid in periodically repeating microchannels. This induced flow, in turn, creates a synthetic gauge field that gives rise to topological sound waves. An alternative way to generate synthetic gauge fields in solid media relies on applying mechanical strains. We show that a suitably strained mechanical metamaterial can exhibit an acoustic spectrum with discrete Landau levels, in analogy with an electron gas in a magnetic field. Our findings pave the way for engineering acoustic devices with strain-induced tunable band gaps.
Burak Eral (TU Delft) (15.45-16.15)
Engineering particle trajectories in microfluidic flows and Composite hydrogels for crystallisation
In this talk, I will discuss two ideas where hydrodynamics and soft materials are utilised to bring out-of-the-box manufacturing ideas. In part I, I will discuss how fundamental understanding of quasi-two-dimensional hydrodynamics can be exploited to control particle trajectories in microfluidic flows. In part II, I will describe a valorised process where unique properties of soft matter are leveraged to design continuous manufacturing methods for fine chemicals industry.
Ties van der Laar (Wageningen UR) (16.15-16.45)
Sticky colloids get jammed
Attractive interactions between colloidal spheres can drastically alter their mode of dynamic arrest, both in quiescent conditions and under flow. Here we discuss two examples in which stickiness affects the dynamics of suspensions. We first describe how attractive interactions can push a repulsive colloidal glass into another amorphous state with very different properties. Upon introducing weak attractions between particles the configurational glass melts and gives way to an attractive glass characterised by physical bonds between neighboring particles. Using 4D microscopy, we show how the onset of cage weakening and bond formation is signalled by subtle changes in local structure. This gives rise to a discontinuous solid-solid transition, which is marked by a critical onset at a threshold bonding energy, exhibiting complex heterogeneous dynamics. We will also show how weak interactions affect the formation of clogs when a suspension flows through a narrow constriction. The effects of flow, constriction geometry and attraction strength on the formation of flow blockades can be quantitatively captured using a simple model based on transition-state theory. These results show how even weak particle interactions can have drastic effects on the formation of arrested states, both in and out-of equilibrium.