Abstracts

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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.

Soundbites

Etienne Jambon-Puillet IOP University of Amsterdam Dynamic of spreading and evaporating droplets: We study the evaporation of droplets of completely wetting liquids deposited on a perfectly smooth surface. Since the liquid is perfectly wetting, the drop starts to spread on the surface as soon as we deposit it. However, since the droplet evaporates at the same time, it starts to recede shortly afterwards, with a constant non zero contact angle. This simple experiment involves two singularities, both the viscous stress and the evaporative flux diverge at the triple point. By examining carefully the microscopic region at the edge of the drop we propose a model that reproduce our experiments quantitatively.
Milo Stroink IoP University of Amsterdam Self assambled Quantum dots on a surface: Using the critical Casimir effect we are self assembling Quantum dots on a surface. We have found that that we can influence the assembly of the dots by changing the interaction of the Casimir by temperature. We have also found dots growing in islands on the surface of silicon.
Robbie Rens IOP, UVA Elasticity of submarginal networks with rigid-backbones: Many types of polymer are known to be very flexible towards bending and rather stiff towards stretching. The separation of these energy scales can give rise to interesting mechanical behaviour in networks build from these polymers. Submarginal networks with only central-force interactions are known to be floppy towards small deformations, but additional interactions can stabilize the network. Weak interactions typically couple to floppy modes, giving rise to a sharp stiffening of elastic moduli upon large deformations, reminiscent of the diverging susceptibility in second order critical points. In order to properly understand the criticality observed in these strain stiffening networks, we formulate a theoretical framework which allows us to exactly probe the limit of vanishing weak interactions. Our framework provides valuable insights into the micro-mechanics of biopolymer materials. Ultimately, we will able to understand how in such limits criticality is induced by the microscopic details.
Daniel Pearce Leiden University Controlling a turbulent active fluid with geometry: Active nematic liquid crystals are achieved experimentally by combining high concentrations of microtubules, kinesin and ATP. These active nematic suspensions can then be localized to a 2D interface, such as the surface of a water droplet suspended in another medium. We will discuss the distribution of topological defects within an active nematic liquid crystal confined to the surface of a toroidal water droplet. Due to the topology of the surface, it is possible to coat a torus in a nematic material with no topological defects. When the activity of the nematic is sufficiently high, topological defects will spontaneously form, but they must preserve the net topological charge of the system, which is zero. Through experiment and simulation we confirm that the local distribution of topological defects is proportional to the Gaussian curvature. As the activity of the system is increased the reliance on the Gaussian curvature is diminished and the available area on different regions of the torus dictates the behavior.
Vera Meester Leiden University Patchy colloids of deformable seed particles: To build complex structures complex building blocks are required. Micron-sized particles with an anisotropic shape and patchy interactions are often referred to as ideal building blocks due to their complexity and opportunities for self-assembly. We present a wide variety of patchy particles prepared by the reconfiguration of random aggregates of spheres induced by addition of organic solvent, the so-called “colloidal recycling” method. During this process strong capillary forces act on the seed particles in the cluster. Soft spheres of low elasticity are observed to strongly deform by these forces, while the shape of elastic spheres is largely preserved in the resulting patchy particles. We tune the elasticity of the seeds by variation of the crosslink density of polymer particles. For clusters constructed by six spheres the deformability of the seeds also influences the reconfiguration pathway and therefore the final geometry obtained. Besides one-component systems, we exploited the colloidal recycling method using binary mixtures of colloids of different sizes and rigidness, resulting in fascinating patchy particles.
Koen Schakenraad Leiden University Cytoskeletal anisotropy controls geometry and forces of adherent cells: We introduce a simple mechanical model for adherent cells that quantitatively relates cell shape, internal cell stresses and cell forces as generated by an anisotropic cytoskeleton. We perform experiments on the shape and traction forces of cells with an anisotropic cytoskeleton, cultured on microfabricated elastomeric pillar arrays. We demonstrate that the shape of the cell edge between focal adhesions is accurately described by elliptical arcs, whose eccentricity expresses the ratio between directed and isotropic stresses. Our work paves the way toward the reconstruction of cellular forces from geometrical data available via optical microscopy.
Yujie Zhou Leiden University Vibrational modes in twisted Kagome lattices: tbd
Alvaro Marin University of Twente Particle trajectory entanglement in micro-confined channels: Suspensions in motion can show very complex and counterintuitive behavior, particularly at high concentrations. However, simpler behavior is always expected at low particle concentrations. When dilute particle solutions (typically below 10% relative volume) are forced through confined channels, particles are expected to flow in straight trajectories, slightly lagging behind the liquid flow velocity accordingly to their finite size. In this very short presentation I will show that, in general, this is not the case and the particle dynamics become much more complex.
Serhii Mytnyk TU Delft Compartmentalizing supramolecular hydrogels using aqueous multi-phase systems: We describe an approach utilizing aqueous multi-phase systems to create separate aqueous micro-compartments in supramolecular hydrogel materials. We found that the self-assembly of supramolecular hydrogelators in 2- and 3-phase all-aqueous emulsions allowed us to prepare soft hydrogels with distinct microstructure. These soft hydrogels consist of different compartments which are not separated by hydrophobic boundaries, and can be addressed individually. Interestingly, the dynamic self-assembly of the supramolecular hydrogelators could be exploited to achieve dynamic compartmentalization. These features offer new opportunities for the design of cytoplasm-mimicking soft materials.
Soumyadipta Sengupta TU Eindhoven Structure and Dynamics of Hydrated Polyelectrolyte Membranes: Polyelectrolyte Membranes (PEM) under hydrated conditions are used in flow batteries and fuel cells. PEM’s separate the catholyte and anolyte in a battery and separate the hydrogen and oxygen streams in a fuel cell. These PEM’s in the hydrated state allows the passage of protons in both fuel cells and flow batteries. The ease of proton passage affects the efficiency of both batteries and fuel cells. Proton conductivity of PEM’s is affected by the size of water clusters and amount of hydrogen bonding. High temperatures, which occur in fuel cells, also reduce proton conductivity due to loss of water from the PEM’s. We are trying to understand the internal structure and the dynamics of proton transport at different temperatures for PEM’s like Nafion and compare it with newly developed PEM’s like Perfluoroimide acid (PFIA). Effect of doping the PEM's with ionic liquids will also be examined. A multiscale approach has been chosen for our study in which we start from classical atomistic simulations and proceed to Lattice Boltzmann simulations. Atomistic simulations have shown the formation of water clusters and hydrogen bonded structures. A comparison of the internal structure and dynamics between PFIA and Nafion has also been done.
Tomasz Krzysztof Piskorz TU Delft Domain formation and dynamics of functionalized long-alkanes on graphite: Controlled 2-D nanopatterning is a promising strategy to create well-controlled nanostructures that might have application in industry, especially in organic electronics. To improve the control over this process a better understanding of its mechanism has to be achieved. In this work, we use coarse-grained simulations of self-assembly of long functionalized alkanes on a graphite surface to give insights into this process.
Jeroen Rodenburg Utrecht University Predicting phase coexistence in active matter?: From a theorists' perspective, active matter fascinates because it's non-equilibrium nature challenges our equilibrium-based intuition. A particular striking example is that purely repulsive colloidal spheres below the freezing density, phase separate when made active, into a gas and liquid-like phase. This pheonomenon is known as motility-induced phase paration. This begs the question whether the phase-coexisting densities can be predicted, as one would do in equilibrium by equating pressures and chemical potentials of the coexisting phases. My research focuses on the concept of pressure and chemical potential for active matter. Whereas one can still (carefully!) define a pressure, the definition of a chemical potential seems to be more problematic. This signals the need for a truly non-equilibrium theory to predict the phase-coexisting densitities.
Shari Finner TU Eindhoven Percolation theory of lyotropic liquid crystals: The design of polymer composites with conducting fillers like carbon nanotubes has a wide range of applications in the electronics and photovoltaics industry. Above a critical particle concentration called the percolation threshold, fillers start forming a system-spanning network which strongly affects the mechanical and transport properties of the material. The percolation threshold of a polydisperse system is known to scale with the inverse weight-average of the particle length distribution. Invoking connectedness percolation theory, we find that this relation does not generally hold anymore in the presence of external alignment fields, such as electric fields, elongational flow fields and molecular fields provided by nematic liquid-crystalline hosts.
Vasudevan Lakshminarayanan Delft University of Technology Modelling kinetics and predicting gel points in an acid triggered supramolecular gel: The ability to comprehend and control microstructure of gel fibers allows us to create materials for drug delivery, tissue engineering and eventually even understand cellular organization. In our contribution, we showed the ability of an acid triggered supramolecular gel to form different architectures under varying rates of change of pH. We investigated the rheology of the obtained gels and observed differences in fractal dimensions which correlate well to microscopic characterization. We explored the applicability of existing fiber network branching models and found that the Avrami model provides the best fit across the multiple investigation strategies in the concentrations explored. Furthermore we created a kinetic model and combined it with a random graph model to predict gel points of the formed supramolecular gel.
Marcel Workhamp Wageningen University Direct effect of interparticle friction in suspensions of soft particles: We experimentally study the role of friction in dense athermal suspensions of soft hydrogel spheres. For some particle types, we observe a flow instability and flow localization in the fluid/particle mix. What creates this instability? To find out, we characterize the interparticle friction between individual gel spheres in a novel experimental design, and also measure the effective friction coefficient of the suspension. We find a direct relation between the individual and collective shear resistance, provided the particle friction coefficient exceeds a critical value. This suggests that suspension flows are dominated by two mechanisms, one of which is directly determined by friction. Since flow instabilities and flow localization are associated with the formation of local structure, we conjecture that friction induces a local microscopic ordering in dense athermal suspensions.
Benjamin Klemm TU Delft Cell based drug delivery devices using photo-polymerizable hydrogel backpacks for cellular hitchhiking.: In this research, we established and defined a novel preparation method for customizable, large quantity production of hydrogel backpacks specifically targeting cellular hitchhiking applications for drug delivery. We introduce a simple fabrication method capable of producing large quantities (~ 1000 objects) of hydrogel patches of various micro-meter sizes (10 - 250 um) and customizable shapes, while achieving both, cell adherence as well as cell proliferation. Hence, obtaining floating objects, which remain structural viable and stable over longer periods of time.
Lucia Baldauf UvA Colloidal superballs in spherical confinement: Packing of objects in confinement is an issue with far-reaching implications from biological pattern formation to information storage. It is already known how spherical particles pack in confinement, and recent computer simulations show how differently perfect cubes pack when confined in small volumes. In this work we use colloidal superballs, particles whose shape interpolates smoothly between cubes and spheres, to experimentally investigate the transition from cube to sphere packing by confining them in evaporating emulsion droplets.
Raoul Frijns TU Delft Delayed crack nucleation in soft polymer networks: Soft polymer networks like gels and elastomers break when submitted to a large enough strain. Above a certain critical threshold the material fails instantly. At lower strains fracture can still occur as a thermally activated process; random thermal motions can cause the already stressed single bonds to momentarily exceed their individual rupture energies, leading to an accumulation of microscopic damage. So far, most fracture research has focused on the propagation of cracks through a material but a microscopic understanding of the processes that lead to the initial nucleation of a delayed crack are not yet understood. Using a range of optical scattering techniques and mechanochemical tools we aim to study what happens to the microscopic dynamics and structure in strained polymer networks just before they crack. We also aim to link these phenomena to structural properties such as heterogeneities in crosslinking density or bond strength. Understanding the mechanics of crack nucleation is an essential step towards developing new, fracture resistant soft materials.
Fanny Trousel TU Delft Chemical signal activation of an organocatalyst enables control over soft material formation: Cells can react to their environment by changing the activity of enzymes in response to specific chemical signals. Artificial catalysts capable of being activated by chemical signals are rare, but of interest for creating autonomously responsive materials. We present an organocatalyst that is activated by a chemical signal, enabling temporal control over reaction rates and the formation of molecular materials. Using self-immolative chemistry, we designed a deactivated aniline organocatalyst that is activated by the chemical signal hydrogen peroxide and catalyses hydrazone formation. Upon activation of the catalyst the rate of hydrazone formation increases 10-fold almost instantly. The responsive organocatalyst enables temporal control over the formation of gels featuring hydrazone bonds, creating materials that can respond to a specific chemical signal. The generic design should enable the use of a large range of triggers and organocatalysts, and appears a promising method for the introduction of signal response in molecular materials, constituting a first step towards achieving communication between artificial chemical systems.
Afshin Vahid TU Delft Pattern formation on anisotropically curved membranes: Biological membranes exhibit a variety of morphologies, from simple spherical liposomes to bewildering complex structures. Examples are the tubular networks in the Endoplasmic Reticulum and perforated sheets in the Golgi apparatus. These structures are home to many inclusions, like proteins. Spatial organization of such proteins is crucial for the dynamic behavior of cellular membranes. Previously, the global shape of the membrane is selected from one of three options: planar, spherical, or tubular. Here, through a numerical approach we show that the background shape has a strong effect on protein self-organization. We find that particles adhered to an ellipsoidal vesicle exploit the curvature variation to self-assemble and form a ring at the mid-plane of the ellipsoid. Our results reveal that purely physical interactions on membranes with anisotropic shapes can drive proteins to preferred regions of cellular membranes. Such physical interactions can underlie biological phenomena like endocytosis and cell division.
Dion Koeze TU Delft Attractive emulsions: a sticky problem: By combining different modelling and simulation techniques, we will study the non-linear elastic response and fracture process in soft polymer networks. In particular, we will address how the network failure depends on the material heterogeneity. For this, we will employ lattice-based models composed of stiff and soft elements and observe how cracks nucleate and propagate. Secondly, by using Brownian Dynamics simulations, we will study how transient (viscoelastic) bonds affect the network fracture toughness. Our predictions will be compared with experiments performed in our group.
Simone Dussi Wageningen University A soft-break in polymer networks: By combining different modelling and simulation techniques, we will study the non-linear elastic response and fracture process in soft polymer networks. In particular, we will address how the network failure depends on the material heterogeneity. For this, we will employ lattice-based models composed of stiff and soft elements and observe how cracks nucleate and propagate. Secondly, by using Brownian Dynamics simulations, we will study how transient (viscoelastic) bonds affect the network fracture toughness. Our predictions will be compared with experiments performed in our group.
Corentin Coulais University of Amsterdam Machine Materials: We will present briefly the activities of the newly formed Machine Materials Laboratory, whose focus is the study of materials, which combine internal architecture and active processes in order to interact with their environment in a programmable fashion.
Simon Stuij UvA Stretching colloidal aggregates: Using optical tweezers we impose a stress on single colloidal aggregates. From simultaneous video microscopy we can track the individual particles and study the resulting deformations and changes in the vibrational spectrum.
Rumen Georgiev TU Delft Flip behaviour of dumbbell-shaped particles at low Reynolds number flow in a microfluidic device: The current work is an experimental study of the hydrodynamic self-interactions exhibited by asymmetric particles in microfluidic devices at low Reynolds number flows. The main aim of the project is to provide insight into and control over the flip behaviour of dumbbells by changing their shape and asymmetry. The results of this study are the first step towards understanding and tuning interparticle hydrodynamic interactions which pave the way to novel advanced separation techniques.
Bart Weber University of Amsterdam Molecular probing of frictional contacts: Amontons’ law introduces a material-dependent constant, the friction coefficient, as the ratio between friction force and normal force. Amontons’ law is commonly explained with the two non-trivial assumptions that both the frictional and normal force depend linearly on the real contact area between the two sliding surfaces. Here, we use a new contact detection method with molecular-level sensitivity and for the first time independently probe these two relations. While the friction force is proportional to the real contact area, we find that this real contact area does not increase linearly with normal force. Contact simulations performed on the identical surface show that the breaking of Amontons’ law is due to plastic hardening of the contacting regions. These new insights into contact mechanics pave the way for a quantitative microscopic understanding of contact mechanics and tribology.