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Invited speakers

Janne-Mieke Meijer (Konstanz, Germany) (10.30-11.15)
Observation of solid-solid transitions in 3D crystals of colloidal superballs
Self-organization in colloidal suspensions leads to a fascinating range of crystal and liquid crystal phases induced by shape alone. Simulations predict the phase behaviour of a plethora of shapes while experimental realisation often lags behind. Here we present the experimental phase behaviour of colloidal superbals, i.e. particles with a shape in between that of a sphere and a cube. We show that by that tuning the shape, size and interactions we can probe the phase behaviour in a region where an enrichment of the phase behaviour is predicted. Using a combination of confocal microscopy and high resolution x-ray scattering we find an even richer phase behavior than predicted as three distinct crystal phases are uncovered. Particularly, we find a solid-solid transition from a plastic crystal phase into two different rhombohedral crystal phases, one with hollow-site stacking while the other possesses bridge-site stacking. We further investigate how the phase diagram depends on the exact superball shape and osmotic pressure in the system and additionally find that a slight softness causes a second solid-solid transition between the two stacking sequences at high osmotic pressures. Our investigation brings us closer to ultimately controlling the experimental superball self-assembly into functional materials, such as photonic crystals.
Nick Tito (TU Eindhoven) (11.15-11.45)
Making polymers with personality: a microscopic view on harnessing molecular motion to design responsive materials
Molecules are always in motion at the microscopic scale. Materials that are designed to exploit this motion in creative ways can adapt themselves gradually to their environment, or suddenly to a stimulus. For example, molecular species that form dynamic crosslinks in polymer gels and networks are the basis for a variety of materials that self-heal, or stiffen when reshaped. Stimulus-responsive molecular ingredients (e.g. liquid crystalline species) can, on the other hand, actually drive deformation in a polymeric network when exposed to light or electric fields. In this talk I will discuss how we are using molecular simulation, and particularly, simplified physical models, to discover microscopic design rules for polymers that dynamically respond to stimulus or deformation.
Liesbeth Janssen (TU Eindhoven) (13.30-14.15)
Swimming on a sphere
Active matter refers to systems whose constituent agents can move autonomously through the consumption of energy. The energy dissipation at the single-particle level leads to an intrinsically out-of-equilibrium state, resulting in complex self-organizing behavior that defies the laws of conventional equilibrium statistical physics. In this talk, I will highlight recent results of particle-resolved computer simulations of active rods moving on a spherical surface. This system exhibits a rich non-equilibrium phase diagram, including a novel self-spinning glass phase that is characterized by strong disorder and persistent collective rotation. By periodically swelling and shrinking the confining sphere, we observe the first signatures of time-dependent aging and rejuvenation in an active glassy material. Overall, these results demonstrate both how concepts of passive glass phenomenology can carry over into the realm of active matter, and how topological constraints may provide new pathways for exploring and controlling the out-of-equilibrium behavior of active and responsive materials.
Karsten Baumgarten (TU Delft) (14.15-15.15)
Nonlocal Elasticity Near Jamming
Classical continuum elasticity is a long wavelength theory — it is blind to microstructure. Hence there must be some length scale where the theory can no longer accurately predict deformations in a discrete system. I will introduce a model-free numerical test to determine this length scale. As an added benefit, the test also provides a new nonlocal constitutive relation, which extends continuum theories to shorter wavelengths where classical elasticity fails. As an example, I will apply these methods to packings of soft repulsive spheres, a widely studied model for the jamming transition. Perhaps surprisingly, the length scale where elasticity breaks down is much larger than the particle size, and diverges at the jamming transition.
Ivan Rehor (Utrecht University) (15.45-16.15)
Light Controlled Hydrogel Microcrawlers
Soft robots are machines constructed from flexible materials connected jointlessly, that exert mechanical work by deformation. This is in contrast to the classical ‘hard robots’ relying on stiff components connected by joints. This fundamental difference possesses several advantages of soft robots, notably lower requirements on precision during construction and operation which is a significant hurdle for miniaturization of machines. Several designs of soft locomotive robots, i.e. crawlers or walkers were reported in the literature1 and show viability of the concept. However, the mechanisms of wireless control and steering as well as miniaturization below current possibilities of ‘hard’ robots are yet to be developed. In this contribution, we describe microsized (on the order of 10 to 100 μm) hydrogel crawlers, remotely controlled and steered by light. The crawlers were prepared using continuous high throughput method Stop-Flow lithography and consist of a thermoresponsive polymer loaded with gold nanoparticles. When irradiated, gold nanoparticles absorb the light and heat up, which leads to local collapse of the thermoresponsive hydrogel. To induce the motion, the rear part of the crawler is irradiated with focused laser pulses, causing periodic contraction and expansion of the irradiated area. The friction between the hydrogel and substrate changes during the contraction/expansion process, exhibiting hysteresis. This hysteresis is responsible for the symmetry breaking between contraction and expansion, resulting in a forward motion of the crawler. While the crawling mechanism is interesting in itself, developed crawlers have also practical applications. More complex crawler shapes allow for steering (turning left, right) and thus can be used as micromanipulators to transport and assemble other micro objects. The developed system is relatively easy to prepare with high throughput methods, and the motion in terms of coupling between light, temperature, friction and local volume is by a fundamentally new mechanism. Previously presented soft crawler designs were inspired by the motion of organisms (earthworm, inchworm). Although our mechanism shares certain features with an inchworm motion it is unparalleled in living nature, showing that not only bioinspired designs are suitable in soft robotics.
Harini Pattabhiraman (Utrecht University) (16.15-16.45)
Quasi-periodic and periodic photonic crystals
Colloidal photonic crystals are excellent candidates for the formation of photonic band gaps in the visible region because of their suitable sub-micron particle size and relative ease of formation by self-assembly. This talk deals with such colloidal two-dimensional quasicrystals and three-dimensional periodic photonic crystals. We model the system of colloidal particles exhibiting a core-corona architecture using a hard core plus a repulsive square-shoulder interaction potential. I will first focus on the two-dimensional system. Firstly, we map out the phase diagram of the system at various shoulder widths using Monte Carlo simulations. We find stable quasicrystals of different symmetries, including a dodecagonal quasicrystal at a shoulder width 1.4 times the hard-core diameter (J. Chem. Phys. 143, 164905, 2015 & 146, 1114901, 2017). Secondly, we show that the formation of this quasicrystal is robust over a range of shoulder widths and insensitive to the shape of the interaction potential. Lastly, using band structure calculations we establish that this quasicrystal exhibits a photonic band gap. I will then proceed to the three-dimensional system where we find the formation of a stable pyrochlore lattice at a shoulder width 2.1 times the hard-core diameter. The structure of the pyrochlore formed in our simulations has two inherent length scales and displays complete photonic band gaps in both the direct and inverted crystal structure.