Of all the secrets that Nature has concealed by her method of deadening curiosity by leaving them exposed, in this her method has been the most successful

Osborne Reynolds, 1899

The famous Osborne Reynolds (yes, from the Reynolds number) wrote the lines above with the slipperiness of ice in mind. Yet he might have been discussing many other things: everyday phenomena are rich in interesting and novel physics. Be it the way liquid droplets attach to a soft substrate, the way potatoes become crunchy when you fry them, or the way sand flows: all are just examples of the fascinating physics in materials of everyday life. They often feature unresolved puzzles (the slipperiness of ice still being one of them) and regularly open doors to unexplored rooms of physics. Indeed, the richness of observations is often due to the interplay of various basic physical mechanisms at “miniature” length scales, such as viscous effects, friction, surface tension, adhesion and electrostatic interactions. Although much of this seems “physics”, there is often a surprising amount of “chemistry” involved in such processes, so for me these discipline boundaries are not very sharp.

The Multiphase Matters group of Joshua Dijksman digs deeper into these issues. The main focus is to understand how the interaction among components in heterogeneous media can generate new behavior that one would not expect based on the ingredients alone. Think of how particles in a fluid can make the resulting suspension become rigid [1, 2, 3], or how particles in a gel can make it more or less fracture resistant [4], or how oil droplets in an emulsion can make the emulsion flow more easily [5]. Other examples come from the natural world: active particles together generate complex flow behavior that one wouldn’t have guessed to see from the way the particles are. Birds in a flock are an example, and it one can study such behavior with simplified models in the lab [6] or even with worms in a rheometer. Such large-scale or “macroscopic” behavior appearing from simple microscopic ingredients is also sometimes called “emergent” behavior [7]. Particle interactions are often very relevant, so a substantial effort of the group goes into doing work on friction [8] and adhesion also in the context of a Soft Robotics consortium, in collaboration with Marleen Kamperman. Apart from previously cited work there is much more going on; one area of attention is the mechanical behavior of sand, more technically granular media, including the role of fluctuations in flowing granular media [9] and the compressibility of static packings of soft particles [10, 11] or of the flow behavior of such particles [12]. Much granular work is also happening in the H2020 consortium CALIPER, of which Joshua is the coordinator.

The main focus of the group is evidently on “athermal” systems, but multiphase systems that straddle the boundary of thermal and athermal are also in the crosshairs [13]. We employ an array of experimental techniques, such as rheology, DMA, three-dimensional imaging [14], photo elasticity [15], 3D printing and advanced image analysis [16], to probe the extremely diverse behavior of these fascinating materials. Numerical work is also done on thin films [17,18, 19] where often the multitude of physico-chemical forces acting on the thin films make them surprisingly difficult to understand. Even the somewhat old school MacArthur models pack a punch in their ability to show very relevant emergent behavior in evolutionary dynamics if you simplify and expand them [20].

Evidently this work is often done in collaboration with other people and finds its way into teaching at both the University of Amsterdam and Wageningen University.

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