Abstracts

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Invited speakers

Wilhelm Huck (Radboud University, Institute for molecules and materials) (10.30-11.15)
Cells and gels: matrix elasticity and spreading dynamics regulate cell behaviour
The mechanical properties of the extracellular matrix (ECM) have emerged as an important microenvironmental cue regulating cell spreading and cell fate decisions. Studies on mechanically tuneable synthetic substrates, such as polyacrylamide hydrogels coated with collagen or other ECM proteins, have demonstrated a strong correlation between hydrogel stiffness and cell adhesion, spreading, proliferation and differentiation. In this talk, I will show results of epidermal stem cell differentiation on a range of soft substrates as well as on controlled cellular patterns that limit collective behaviour, and dissociate the different chemical, mechanical and topological parameters involved in dictating cell phenotype. A crucial aspect that has received little attention thus far is the dynamic nature of how complex systems such as cells adapt to their environment. Complex networks could well share some characteristics even if the system as a whole is in a different state, and therefore the cellular response to mechanical properties might not be known when only looking at single observables as cell spreading, cell shape or lineage selection, at a single point in time. I will discuss unpublished findings, where wel demonstrate how hMSCs adapt to different substrates via different trajectories, resulting in steady state behaviour that is similar in certain aspects, but leads to fundamentally different cell states.
Indrani Chakraborty (University of Leiden) (11.15-11.45)
Colloidal Joints with Designed Motion Range and Tunable Joint Flexibility
The miniaturization of machines towards the micron- and nanoscales requires the development of joint-like elements that enable and constrain motion. We present a facile method to create colloidal joints, that is, anisotropic colloidal particles that control the motion range of bonded particles. We demonstrate quantitatively that we can control the flexibility of these colloidal joints by tuning the DNA linker concentration in the bond area. We show that the colloidal shape controls the range of motion that these colloidal joints enable, due to the finite-sized patch of bonded linkers that cannot cross regions of high curvature. Finally we demonstrate the potential of the colloidal joints for programmable bottom-up self-assembly by creating flexible colloidal molecules and colloidal polymers. The reconfigurability and motion constraint offered by our colloidal joints make them promising building blocks for the development of switchable materials and nanorobots.
Bas Overvelde (Amolf) (13.30-14.15)
Embracing Compliance and Instabilities to Achieve Function in Mechanical Metamaterials and Devices
The use of soft materials has led to the development of soft devices that have the potential to be more robust, adaptable, and safer for human interaction than traditional rigid systems. State-of-the-art developments push these robotic systems towards applications such as soft rehabilitation and diagnostic devices, exoskeletons for gait assistance, grippers that can handle diverse objects, and electronics that can be embedded in the human body. Furthermore, compliance has found its way into the design of metamaterials. Applications of compliance in metamaterials range from tunable auxetic behavior, stiffness, optical properties and phononic and acoustic band-gap behavior to tunable surface properties such as the drag coefficient, and chemistry. These examples illustrate the potential of using compliance to create new and improved functionality in structural and robotic applications. While the geometrical non-linearities and instabilities that arise when using soft or flexible materials directly complicate the design process and fall outside the scope of traditional engineering, exactly these non-linearities make the systems inherently capable of rich behavior. As such, to bring these systems closer to application and to uncover their true potential, we need to gain a better understanding of the principles that govern their behavior. In this talk I will focus on the design of non-linear structures and devices - including origami-inspired reconfigurable architected materials and soft actuators that harness instabilities - that exhibit a nontrivial relation between input and output (i.e. loading and response). I propose computational and relatively simple experimental techniques that allow us to effectively explore the design space, and that lead to an understanding of the relation between shape and function in these compliant systems. I will also briefly touch upon "Soft Robotic Matter", our future research direction that focuses on the role that feedback between sensing and actuation could play in these systems, and how we plan on incorporating this feedback in artificial materials.
Natalia Domeradzka (Wageningen) (14.15-14.45)
Cross-linking and bundling of self-assembled protein-based polymer fibrils via heterodimeric coiled coils
Previously, we developed triblock protein polymers that form fibrillar hydrogels at low protein polymer concentrations (denoted C2-SH48-C2). We here demonstrate that the structure of these hydrogels can be tuned via heterodimeric coiled coils that cross-link and bundle the self-assembled protein-polymer fibrils. We fused well-characterized, 47 amino acids-long heterodimeric coiled coil "linkers" (DA or DB) to the C-terminus of the triblock polymer. The resulting C2-SH48-C2-DA and C2-SH48-C2-DB polymers, were successfully produced as secreted proteins in Pichia pastoris, with titers of purified protein in the order of g L-1 of clarified broth. Atomic force microscopy showed that fibrils formed by either C2-SH48-C2-DA or C2-SH48-C2-DB alone already displayed extensive bundling, apparently as a result of homotypic (DA/DA and DB/DB) interactions. For fibrils prepared from protein polymers having no linkers, plus a small fraction of polymers containing either DA or DB linkers, no cross-linking and bundling was observed. At these same low concentrations of linkers, fibrils containing both the DA and the DB linkers did show cross-linking and bundling as a consequence of heterodimer formation. This work shows that coiled coil modules can be employed to control bundling of supramolecular fibrils, which is promising for the further development of materials that mimic the extracellular matrix.
Anupam Pandey (University of Twente) (15.45-16.15)
Liquid drops attract or repel by the inverted cheerios effect
We study the interaction between liquid drops on a soft substrate. When millimetric liquid drops run down a soft, vertical surface under gravity, we observe A deviation of their trajectories from straight line as two drops approach each other. Remarkably, if the wall is very thick drops always attract and coalesce, whereas drops on a thin layer repel each other. We experimentally determine the force-distance curves of this interaction, and perform a free energy minimisation to reveal the underlying mechanism of drop-drop. Importantly, we show that the interplay between capillarity and bulk elasticity of the soft substrate governs the nature of the interaction force.
Viktoria Wollrab (Amolf) (16.15-16.45)
Self-organization of active motor-driven actin-polymer networks
Active cell shape transformations during migration and division are driven by an active polymer gel that is based on a network of actin filaments. Actin filaments possess a structural polarity and interact via passive crosslink proteins and active myosin motor proteins. It is still unclear how interactions of actin filaments and motors on the molecular scale result in collective force generation at the scale of the cell. To elucidate the microscopic basis of the out-of-equilibrium properties of this key biological system we reconstitute biomimetic model systems from purified proteins. We observe a rich variety of self-organized patterns and dynamics that emerge from an initially random state. Interestingly, we find that myosin motor activity results in polarity sorting of actin filaments. Due to directed myosin motion, these networks efficiently generate stresses which lead to contraction. We show that specific motor-filament interaction nucleates these polarity sorted structures while crosslinking stabilizes them and allows their growth. This mechanism might also be relevant for contractility and pattern formation in cells.

Soundbites

Ties van de Laar

Wageningen University
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Ruben Higler

Wageningen University
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Nicholas Tito

TU/e
Discovering Microscopic Design Rules for New Materials
The ability to construct new materials out of microscopic building blocks is still in its infancy, mainly because the pathways by which they assemble are complex. Developing materials therefore relies on discovering microscopic design principles, often inspired by what evolution has created in biological systems. My work invokes theory and simulation to unearth new microscopic "design rules"---that is, how the interactions between molecules can be exploited to create exciting new materials. Recent efforts have focused on "multivalent" interactions---those mediated by many ligand-receptor bonds---and how this leads to superselective binding on targets (e.g. cell membranes or artificial surfaces). Upcoming research will apply these concepts to predicting the properties of polymeric materials with reversible bonds, whilst exploring new models for dynamic "photo-actuation" of related systems.

Daniel Pearce

Leiden University
Active liquid crystals on curved surfaces
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.

Shari Finner

TU/e
Percolation Theory of Lyotropics
We investigate the formation of a system-spanning network of rod-like colloids in solution. The particle concentration at which this happens, the so-called percolation threshold, depends crucially on the rod orientations as well as their level of polydispersity, and experimental values vary by orders of magnitude. Connectedness percolation theory is an integral equation theory that provides an excellent starting point to tackle this problem theoretically.

Jaroslaw Paturej

Leibniz Institute of Polymer Research
Molecular structure of bottlebrush polymers in melts
Bottlebrushes are fascinating macromolecules that display an intriguing combination of molecular and particulate features having vital implications in both living and synthetic systems, such as cartilage and ultrasoft elastomers. However, the progress in practical applications is impeded by the lack of knowledge about the hierarchic organization of both individual bottlebrushes and their assemblies. Herein we delineate fundamental correlations between molecular architecture, mesoscopic conformation, and macroscopic properties of polymer melts. Numerical simulations corroborate theoretical predictions for the effect of grafting density and side-chain length on the dimensions and rigidity of bottlebrushes, which effectively behave as a melt of hairy and flexible filaments. These findings provide quantitative guidelines for the design of novel materials that allow architectural tuning of their properties in a broad range without changing chemical composition.

Casper van der Wel

University of Leiden
Automatic tracking of colloidal clusters
We present a method for the tracking of features that partially overlap, in order to be able to track so-called colloidal molecules. Our approach implements two improvements into existing particle tracking algorithms. Firstly, we use the history of previously identified feature locations to successfully find their positions in consecutive frames. Secondly, we present a framework for non-linear least-squares fitting to summed radial model functions and analyze the accuracy (bias) and precision (random error) of the method on artificial data. We find that our tracking algorithm correctly identifies overlapping features with an accuracy below 0.2% of the feature radius and a precision of 0.1 to 0.01 pixels for a typical image containing overlapping features.

Vera Meester

University of Leiden
Spherical, Dimpled and Crumpled Hybrid Colloids with Tunable Surface Morphology
Surface morphology is a tool to tune physical properties of colloidal suspensions such as the wettability, viscoelasticity and depletion attractions. Existing synthesis methods to obtain colloids with a rough surface morphology often result in colloids with non-tunable surface properties. Here, we developed a synthetic approach to obtain both spherical and shape-anisotropic hybrid colloids with tunable surface morphology. With our approach monodisperse linear polystyrene colloids, obtained in large quantities using a dispersion polymerization method, are swollen and crosslinked with styrene and 3-(Trimethoxysilyl)propyl methacrylate (TPM) in the presence of the polymerization inhibitor hydroquinone. We show that by varying only two experimental parameters, the concentration of the inhibitor and of TPM during swelling linear polystyrene colloids, three different types of particles can be synthesized. Additionally, we demonstrate that all particles can be used as templates for silica coating, resulting in electrostatically stabilized silica-coated hybrid colloids or silica shells with rough, smooth, dimpled or crumbled surface morphology.

Federica Burla

Amolf
Deformation of collagen networks upon repeated cycles of stress
The extracellular matrix plays a crucial role in regulating communication, growth and differentiation of the cells. One of its main component is collagen, a network-forming rigid protein which confers structure and mechanical rigidity to tissues. Here, we investigate the response of in vitro reconstituted collagen networks to repeated cycles of stress. While the network shows an instantaneous hardening response with consecutive cycles, once a waiting time is introduced between each cycle the network recovers its original response. These results might be consistent with a remodeling of the network and formation of temporary bonds between collagen fibrils which come loose over time. These processes are likely to play a role on the movement of cells through the extracellular matrix.

Ruben Higler

Wageningen University
Binary charged colloidal systems
Colloidal systems made up of particles of opposite charge present a very rich phase behaviour. We combine experiments and simulations to explore this system and study the transition from a sparse colloidal percolating gel network to a binary crystal phase; when we change the particle charge ratio and screening length of the system.

Karsten Baumgarten

TU Delft
Tuning elastic moduli in random spring networks
We tune elastic moduli by applying pre-stress to disordered spring networks both above and below isostaticity. In the latter case tensile stresses rigidify initially floppy networks. Tuning is most effective close to and below the isostatic state, where critical effects enhance the susceptibility to external fields.

Lochem Bronkhorst

WUR-PCC
Heterogeneous dynamics in colloidal gels
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Anja Schröder

WUR
Pickering emulsions stabilized by solid lipid particles
This project focuses on the development of physically and chemically stable food emulsions containing high levels of polyunsaturated fatty acids and low levels of (synthetic) antioxidants based on a novel colloidal-scale design using submicron-scale lipid based particles adsorbed at the oil-water interface.

Jan Maarten van Doorn

Wageningen University
Multi-length-scale fatigue in heterogeneous colloidal solids
Solids tend to weaken upon exposure to repeated stresses that are smaller than their yield stress. This procces, known as fatigue, results in serious damage to various materials. In heterogeneous solids stresses are distributed unequally adding a structural dependency to the already complex mechanical response. Here we use colloidal gels as a model for heterogeneous solids. Colloidal gels are non-equilibrium networks of attractive colloids. Roughly these systems can be characterized by two length-scales: the strand level, consisting of aggregated colloids, and the network level which consists of percolated strands. By combining LAOS rheology and BD simulations we are able to disentangle the processes underlying fatigue. Features at the different length-scales respond differently to the applied stress. The non-linear response mainly originates from local deformations in the strands themselves and the linear response is explained by reversible deformations at the network level.

Thomas Kodger

UVA
Fracturing a colloidal solid
New insights into fracture mechanical have recently been obtained thanks to the use of soft polymeric materials with Young's moduli, E<1MPa, which slow down the velocity with which the propagating fracture tip travels through the material. Fast cameras can directly follow the tip velocity, trajectory and their deviations from theory; however single failure events still occur on the molecular length scale. Here we develop a soft colloidal solid which dramatically increases the length scale of the individual failure events to micrometers and follow this process using confocal microscopy. We use a microfluidic device to hydrolytically fracture a cohesive colloidal solid and follow the local and far-field spatial dynamics of the material during a fracture event.

Pieter de Visser

Wageningen University
Exploring the Frictional Behaviour of Hydrogels
Hydrogels are polymeric materials that consist of up to 98% water. They are generally slippery, but not frictionless. The frictional behaviour of hydrogels differs from that of most other materials; for example, they do not follow the Coulomb-Amonton friction model. Much recent work has been dedicated to discovering the origin of this peculiar frictional behaviour. However, still little is known about the frictional contact between two hydrogel surfaces. Here, we experimentally investigate friction between two surfaces of hydrogel using a rheometer. We perform measurements using a custom-made experimental setup. We attach a 3D printed arm to the rheometer axis, in which we fit a hydrogel with a hemispherical cap. We can thus make the hemispherical cap slide over a flat substrate. This setup allows for precise measurement of the frictional behaviour of hydrogels and the influence of the load, sliding velocity and material properties on this behaviour. We find a substantial influence of the sliding velocity on the friction coefficient in a model hydrogel system.

Nitin Singh

Amolf
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Jochem Bronkhorst

WUR-PCC
Heterogeneous dynamics in colloidal gels
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Dion Koeze

TU Delft
Squares and rectangles have a different shear modulus near jamming
Finite size effects in jammed soft sphere packings have been studied in square boxes, but what happens for boxes with an aspect ratio that is not unity? We see a clear effect of having one short box side while the number of particles is large enough to be in the thermodynamic limit according to existing theory. This suggests an extension of our understanding of finite size effects is necessary.

Sanne van Loenen

University of Amsterdam
Non-linear response of colloidal aggregates
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Arjan Boerma

University of Groningen
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Yujie Zhou

Lorentz Institute
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Melissa Rinaldin

Lorentz Institute and University of Leiden
Membrane phase separation on colloids
A strong coupling between membrane geometry and organization of lipid domains has been shown in studies with multicomponent giant unilamellar vesicles (GUVs). In particular, the local curvature can favour segregation of lipids and the presence of lipid domains can drive formation of curved regions. However, in vesicles these these two effects cannot be disentangled.In order to unravel how composition and curvature are correlated, we have developed a method that allows to fabricate membranes with prescribed geometry. To do so, we create a supported lipid bilayer on colloidal particles and microstructures of different shapes, including superballs and nano-flowers. By varying the bending modulus of the membrane and the curvature of the substrate we experimentally show the effect of the curvature-composition coupling on phase separation patterns.

Mathijs Vermeulen

TU Eindhoven
States of self stress in the Mikado Model
The Mikado model is a widely used model to study the mechanics of fibrous networks. Although widely used, it does not capture an import feature of physical networks, namely states of self stress. I shall show why.

Anton Souslov

University of Leiden
Odd viscosity in compressible active liquids
As a class of fluid materials, active liquids combine a unique set of characteristics. We explore the consequences of one such characteristic - Time-Reversal Symmetry (TRS) breaking - in a liquid consisting of active particles that rotate spontaneously. Such active-rotor liquids maintain isotropy which, in combination with TRS breaking, leads to one extra component in the viscosity tensor of these two-dimensional liquids. This so-called odd (equivalently, Hall) viscosity provides no energy dissipation, but gives rise to a transverse response - for example within a hypersonic shock. We examine how such transverse response can be used to couple rotational motion and linear stresses to create the liquid analog of a mechanical crank.

Piermarco Fonda

Lorentz Instituut
Modeling phase separation on curved surfaces
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