conditions inside the particle, for example by changing the solvent quality with temperature, or adjusting the charge density with pH can thus give rise to changes in the particle size. [ 1 ] Similarly, increasing the particle concentration in bulk solutions can induce osmotic compression of the particles and lead to a size reduction. [ 2 ] By contrast, microgel particles display much of the same characteristic phase behavior that solid, dispersed, colloidal suspensions exhibit, such as crystallization, [ 3 ] glass formation [ 4,5 ] and gelation.[ 6 ] Microgels thus display a fascinating polymer-particle duality [ 7,8 ] ; this is refl ected in, for example, the particle-like scaling of rheological data at intermediate concentrations, and the polymer-like behavior in dense systems. [ 9 ] While recent work has established the versatility of microgels in stabilizing fl uid interfaces, [10][11][12][13][14][15][16][17][18][19][20][21][22] the details of their adsorption, conformation and organization remain largely elusive. Moreover, how their compressibility and deformability interplay with the other forces that act on particles at a liquid interface is still largely unknown. Cryo-SEM imaging, upon freeze-fracturing of microgel-stabilized emulsions has shown that signifi cant deformation of the soft particles may occur. [ 13,14,20 ] This suggests a subtle interplay between the forces acting on the particles at the interface, such as capillary forces and the internal elasticity of the microgels. However, due to the thermosensitivity and delicate nature of microgels this information would ideally be obtained from non-invasive methods that do not require extreme temperature changes or phase changes in the two immiscible liquids. Direct and accurate imaging of solvent-swollen microgels is diffi cult as their refractive index is inherently close to that of the aqueous solvent that swells them. Moreover, addition of a fl uorescent dye, even in small amounts, can signifi cantly alter their interfacial properties.Composite microgels effectively stabilize oil-water interfaces. [ 23,24 ] In this paper, we prepare composite microgel particles in which a highly fl uorescent solid core is embedded into a pH and temperature responsive microgel shell; this design allows accurate and in-situ visualization of the structure of microgel-laden fl uid interfaces. Combining direct in-situ observation with tensiometry allows us to estimate the adsorption energy and the particle elasticity. We place our observations Microgel particles display an interesting duality with properties attributed typically both to polymeric and colloidal systems. When adsorbed at a liquidliquid interface, this duality becomes particularly apparent as the various phenomena at play are governed by different aspects of these soft and responsive particles. The introduction of a solid, fl uorescently labeled, polystyrene core into the microgels allows direct and accurate visualization without the necessity of potential perturbing sample preparation techniques. By com...

We present a complete toolbox to use responsive ionic liquid (IL) emulsions for extraction purposes. IL emulsions stabilized by responsive microgels are shown to allow rapid extraction and reversible breaking and re-emulsification. Moreover, by using a paramagnetic ionic liquid, droplets can be easily collected in low magnetic fields.

A photoelastic material will reveal its internal stresses when observed through polarizing filters. This eye-catching property has enlightened our understanding of granular materials for over half a century, whether in the service of art, education, or scientific research. In this review article in honor of Robert Behringer, we highlight both his pioneering use of the method in physics research, and its reach into the public sphere through museum exhibits and outreach programs. We aim to provide clear protocols for artists, exhibit-designers, educators, and scientists to use in their own endeavors. It is our hope that this will build awareness about the ubiquitous presence of granular matter in our lives, enlighten its puzzling behavior, and promote conversations about its importance in environmental and industrial contexts. To aid in this endeavor, this paper also serves as a front door to a detailed wiki containing open, community-curated guidance on putting these methods into practice (Abed-Zadeh et al. in Photoelastic methods wiki https ://git-xen.lmgc.univ-montp 2.fr/Photo Elast icity /Main/wikis /home, 2019).

We perform experiments on an active granular material composed of individually-driven, spinning disks confined within a circular arena. Small bumps at the outer edges of the disks provide a variable amount of interparticle coupling in the form of geometric friction. The disks each spin counter-clockwise, but undergo a transition in their collective circulation around the center of the arena, from a clockwise orbit to a counter-clockwise orbit, as a function of packing fraction φ. We identify that, unlike for vibrated granular gases, the particles' velocity distributions are Gaussian over a large range of φ. By fitting the speed distribution to a Maxwell-Boltzmann distribution, we identify a temperature-like parameter which is a universal function of φ; this parameter is also equal to the mean translational energy of the particles. We quantify the collective circulation via its solid-body-like rotation rate, and find that this is a universal function centered around a critical packing fraction. In addition, the ratio of orbital kinetic energy to spin kinetic energy is also a universal function for non-zero geometric friction. These findings highlight the important role of both the type of driving and the interparticle interactions (here, geometric friction) in controlling the collective behavior of active granular systems.

We describe a method to produce millimeter-sized hydrogel particles, by dispersing aqueous droplets in an oil using a nozzle and subsequently solidifying them. We show that we can vary the size of the particles using an air flow along the nozzle. The resulting particle size can be well predicted by a simple model where a drag force generated by the air flow, adds to the weight pulling the droplet from the nozzle. Particles produced using this method have diameters ranging from 0.7 to 2.3 mm. Production rates up to 0.5 ml/min per nozzle have been achieved, which compares favorably to standard microfluidic techniques. Finally, we show that the method can be used to produce both physical and chemical gel particles and is thus highly universal.