Abstracts

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Invited speakers

Joost de Graaf (University of Utrecht) (10.30-11.15)
The Impact of Hydrodynamics on Colloidal Gel Collapse
Many industrial systems require micron-sized particles, so-called colloids, to remain suspended in a liquid medium at low volume fractions, e.g., pesticides, beauty products, and paints. To achieve the desired shelf life, one must postpone phase separation and reduce the tendency of density-mismatched colloids to sediment to the bottom of a container. This is often done by introducing strong, short-ranged attractions between the colloids using polymer-based depletion, which lead to kinetic arrest of the phase-separating suspension. That is, the particles become stuck to each other and form a space-spanning network structure, a colloidal gel. This network can provide protection against applied stress, whilst allowing for flow, and it can even exhibit self-healing properties. In this presentation, we introduce an experimental model system, by which colloidal gelation and gel collapse - the eventual break-down of stability under the influence of gravity - can be studied. From the experimental studies , it becomes clear that fluid flow plays an important role in determining the stability of colloidal gels. This inspired us to perform detailed molecular dynamics simulations accounting for hydrodynamic interactions via the lattice-Boltzmann algorithm to gain a better understanding of these systems. We reveal how hydrodynamic interactions can lead speed up and slow down colloidal gel collapse, depending on the colloid volume fraction. We will also comment on the relation between our numerical results and the experiment, as well as those of other groups working in this field. Finally, we present an outlook for future research into gelation that goes beyond the limitations of our current work.
Huanshu Tan (University of Twente) (11.15-11.45)
Evaporating/dissolving multicomponent drops with the ouzo effect
Ouzo is an anise-flavored aperitif, primarily consisting of water, ethanol and anethole oil. Diluting the solution with water causes oil phase to separate out generating long-lived small oil droplets spontaneously, and this process is known as the ouzo effect. We introduced the ouzo effect into multi-component drop systems by letting sessile ouzo drops evaporate in the air or dissolve in liquid. The outcome is amazingly rich thanks to the complexity of the system, and we discovered many interesting phenomena. For instance, the ouzo effect could be triggered by evaporation/diffusion, and the preferential location of oil droplets nucleation is tunable; The nucleated oil droplets could merge up, forming an oil ring sitting at contact line or suspending inside the drop; In different cases, the flow in the system had different patterns. In this talk, I will present and discuss these phenomena.
Corentin Coulais (University of Amsterdam) (13.30-14.15)
From Mechanical Metamaterials to Machine Materials
Manipulating physical signals such as light, sound, heat or motion is a vital challenge in multiple areas of science, with far-reaching ramifications for technology and society. Due to recent advances in digital fabrication techniques, the last years have seen a revolution in artificial periodic composites with on-demand electromagnetic, acoustic, thermal and mechanical properties that surpass that of their constituents and that have important applications in e.g. telecommunications, energy management and medicine. These so-called metamaterials become particularly interesting in mechanics, where geometrical effects, nonlinear responses and coupling to the environment are much more accessible and stronger than in any other physical field. In this talk, I will show that mechanical metamaterials using geometrical nonlinearities and activity lead to entirely new properties and functionalities such as programmable and self-oscillatory responses. Using 3D printing of flexible materials, precision desktop experiments, numerical modelling and theory, we demonstrate that flexible metamaterials can be designed, fabricated and programmed for specific mechanical tasks. Such approach opens up promising pathways to bridge the gap between Matter and Machine.
Abheeti Goyal (TU Eindhoven) (14.15-14.45)
Impact of Substrate Wetting on Domain Morphology and Demixing Dynamics of Binary Fluid Mixtures
Understanding of the evolution kinetics of the demixing of fluids on a substrate is important in the context of the fabrication of membranes, photonics and photovoltaics. By means of lattice Boltzmann simulations we study the structural evolution of binary fluid mixtures undergoing demixing by spinodal decomposition on a substrate that preferentially wets one of the components. We find that this preferential wetting significantly affects the demixing dynamics and morphology both normal and parallel to the substrate. A concentration wave emerges at the substrate, leading to a continuous domain growth normal to the substrate that is much faster than what happens in bulk. We find non-monotonic domain growth in the direction along the substrate, resulting from the eventual breakdown of a bi-continuous morphology into droplets. The time evolution of the length scales normal and along the substrate are found to be linked. The early stages of demixing are significantly affected by the imposed initial noise level, while the late stage coarsening we find to be dominated by hydrodynamics.
Michele Zanini (University of Utrecht) (16:00-16:30)
From liquid interfaces to soft matter: when the single particle surface roughness matters
Surface roughness significantly affects many properties of colloids, from depletion and capillary interactions to their dispersibility and use as emulsion stabilizers. Here, we synthesize a library of all-silica microparticles with uniform surface chemistry, but tuneable surface roughness and we study both their adsorption at oil-water interfaces and the effect of nanoscale surface roughness in linking the rheology of dense suspensions to the particle-particle tribology. In the former case, we demonstrate that surface roughness strongly pins the particle contact lines. As a consequence, the adsorption is arrested in long-lived metastable positions and tremendous contact angle hysteresis are observed. As a unique consequence, the same rough particles can be used as universal stabilizers for both water-in-oil and oil-in-water emulsions by just changing the phase in which they are initially dispersed. When used in dense suspensions, rougher surfaces lead to a significant anticipation of discontinuous shear thickening (DST) onset, in terms of both shear rate and solid loading. Direct measurements of particle–particle friction therefore highlight the value of an engineering-tribology approach to tuning the thickening of suspensions These results both shed light on fundamental phenomena concerning rough particles, indicating new design rules for particle-based emulsifiers and validate surface roughness as an engineering parameter to tune the rheological properties of dense suspensions.
Christian Sproncken (TU Eindhoven) (16.30-17.00)
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Soundbites

Shari Patricia Finner

TU Eindhoven
Percolation in Liquid Crystals
Understanding the formation of clusters within fluid dispersions of rod-like particles is a fundamental problem in soft matter theory, which can help us design novel nanocomposite materials. The formation of finite and system-spanning (percolating) clusters must clearly depend on whether the particles are in the isotropic or in the nematic phase. However, theory and simulations have so far focused on the isotropic phase alone. Using connectedness percolation theory, we show that clusters in the nematic phase are strongly elongated along the director field, and that percolation in the (chiral) nematic almost exclusively depends on the thickness of the contact shel -- a distance criterion which defines whether two particles are connected or not.

Nicholas Tito

TU Eindhoven
Harnessing entropy to enhance toughness in reversibly-crosslinked polymer networks
Materials composed of polymers that are permanently crosslinked into a network, such as gels and rubbers, eventually break if strained enough. This is because the network irreversibly ruptures once the local forces acting on the polymers and crosslinks become too large for the chemical bonds to withstand. Recent experiments have revealed that adding *reversible* crosslinks to the network allows the material to be strained to a much larger extent, yet without altering its elasticity [1]. We are using theory, molecular simulation, and polymer self-consistent field theory for networks [2], to explore reversible crosslinking as a design paradigm for creating polymer networks that are tough but elastic. Emphasis will be placed on how entropy itself drives reversible crosslinks to toughen the material, while preserving its intrinsic elasticity.

Jeremy Ernst

Leiden University
The Mechanical Interplay between Cell Shape and Actin Cytoskeleton Organization
We numerically study the mechanical interplay between the shape of adhering fibroblasts on a micropillar array and the orientation of the actin stress fibers within the cytoskeleton. We model the stress fiber organization as a competition between alignment of stress fibers with one another and with the cell edge. The edge of the cell in turn experiences a contractile force in the direction of local stress fibers. We compare our results to experimental data on fibroblast shape and actin cytoskeleton organization.

Simon Stuij

University of Amsterdam
Patchy colloidal assembly of dimers and tetramers with critical Casimir forces
Patchy dimer and tetramer colloidal particles are synthesize using colloidal fusion [1]. In a close to critical binary mixture, critical Casimir attraction are induced between the patches of these particles. We us the fine temperature control of this interaction to assemble different patchy structures in a reversible manner. Dimers assemble in linear chains. And, at least on short timescales, tetramers assemble in a variety of structures such as corrugated chains, pentagons and branched networks. We study questions regarding the assembly dynamics and structural mechanics of these colloidal superstructures.

Riande Dekker

University of Amsterdam
Emulsion destabilization by confinement
Emulsions play an important role in everyday life and can be found in for example food, cosmetics and paints. The destabilization of emulsions is a key step in for example oil recovery, to extract water from crude oil. Present techniques to destabilize these crude oil emulsions are often energy-intensive and/or use chemical additives that end up in the water [1]. Recently, it was shown that mechanical destabilization during spreading can be used to destabilize emulsions also [2]; however its mechanism remained unclear. We study the destabilization of surfactant-stabilized oil-in water emulsions placed between two glass plates that are pressed together. Using confocal microscopy, we observe the deformation of the oil droplets from perfectly spherical to ellipsoidal. Applying more pressure the sample thickness becomes comparable to the drop size and the emulsion film between the droplets becomes clearly visible. This film thins and eventually breaks, leading to the massive coalescence of droplets occurred and hence the destabilization of the emulsion. This new technique then allows for separation of the oil and water phases from an initial emulsion in a simple manner.

Thijs van der Heijden

TU Eindhoven
Charging of droplets on polymer substrates
Experiments have indicated that droplets acquire a net charge when they are dragged over a polymer substrate. The surface charge of the dragging trail is opposite to the charge at the location of evaporation, suggesting that the droplets take up charges from the surface after deposition and redeposit them during evaporation. Since the problem consists of a complex interplay between the dissociating charges at the surface, the free charges in the liquid and the resulting electric potential, we propose a dynamical model for the charging of droplets based on Langmuir dynamics and diffusion of charges influenced by an electric field.

Karsten Miermans

Arnold-Sommerfeld Center for Theoretical Physics, LMU Munich
SMC Condensin: The Motor behind Bacterial Chromosome Organization?
The bacterial chromosome is highly organized over a variety of length-scales, largely through the action of a highly conserved protein-complex called condensin. How only a few of these condensins can organize a polymer millions of basepairs in length has largely remained elusive. Using a stochastic computational model, we show how the ATPase activity of condensins on the molecular level collectively structures the chromosome over all length-scales.

Martin Brandenbourger

University of Amsterdam
Flatband Non-reciprocal Isolation in Active-Feedback Mechanical Devices
Unidirectional motion isolation and mechanical non-reciprocity are highly beneficial to a wide range of potential applications, ranging from shock and vibration damping to motion assistance and architectural stability. To date, they have been realized using broken spatial or temporal symmetries, yet only in the vicinity of resonances or using nonlinearities, thereby they remain limited to narrow ranges of frequencies or input magnitudes. Here, we introduce mechanical devices enhanced with motors, sensors and computing units, where we program a novel active-feedback mechanism that competes with ordinary reciprocal mechanical interactions. We demonstrate that such meta-atoms can generate huge level of non-reciprocity for an unprecedented broad range of frequencies and amplitudes, with a programmable efficiency controlled by only one parameter.

Paul Baconnier

University of Amsterdam
Contact Mechanics and Friction Vs. Surface Roughness
I will try to answer to the following questions: How to visualize real contact area? How surface roughness affect contact mechanics and friction?

Paul Baconnier

University of Amsterdam
Contact Mechanics and Friction Vs. Surface Roughness

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