Prof. James Harden 

Department of Physics, University of Ottawa

Rheo-XPCS studies of yielding, recovery and memory in soft glassy nanocolloid suspensions

Prof. Dominik Wöll

Institute of Physical Chemistry, RWTH Aachen University, Germany

Super-Resolution Fluorescence Imaging of Microgels

Dr. Tom de Geus

Physics of Complex Systems Laboratory, École Polytechnique Fédérale de Lausanne 

(Why) Are there powerlaw earthquakes?

Laws of friction have been studied since 500 years, yet their microscopic underpinning still eludes us. We do not understand how slip events are nucleated, nor what controls the distribution of their magnitude; questions that are central in earthquake science. The depinning transition has been an attractive theory to think about earthquakes. However, their relation has been questioned, as it has been argued that earthquakes are not powerlaw. 

I provide a novel framework to capture these phenomena and re-establish the link between earthquakes and critical phenomena, by considering continuum descriptions perturbed by disorder. It predicts the existence of power-law distributed slip events whose size diverges at a critical stress, and that nucleate global slip (massive earthquakes, preceded by fracture-like front) if they extend past a critical length. I confirm these predictions in a minimal model of a frictional interface. In addition, the coupling between the flow properties of the interface and criticality yields a prediction for the relationship between the size and the duration of the avalanche (earthquake).

STEFANI Fernando D. ,  Center for Bionanoscience Research (CIBION), National Scientific and Technical Research Council (CONICET), Buenos Aires, Argentina 

 RASTMIN: single molecule localization with 1 nm precision on a confocal set-up 

Localization of fluorophores is key for optical measurements at the nanoscale and beyond ensemble averaging. Prominent examples are single-molecule tracking and super-resolution imaging by single-molecule localization microscopy (SMLM). Among single-molecule localization methods, MINFLUX outstands for achieving a ~10-fold improvement in localization precision over the most popular wide-field camera-based approaches, achieving 1 nm resolution with common fluorophores under ambient conditions. However, its widespread application has been hindered due to its high technical complexity, elevated cost, and, we believe, because it was originally presented in a conceptual framework that may be difficult to grasp at first sight. Here, we will present a new conceptual framework to explain MINFLUX and other related methods such as orbital tracking, and a new method, RASTMIN, that delivers localization precision equivalent to MINFLUX and can be implemented on any scanning (confocal) fluorescence microscope. 

References:

F. Balzarotti, Y. Eilers, K. C. Gwosch, A. H. Gynma, V. Westphal, F. D. Stefani, J. Elf, S. W. Hell

“Nanometer resolution imaging and tracking of fluorescent molecules with minimal photon fluxes”

Science  355 (2017) 606-612

L. A. Masullo, F. Steiner, J. Zähringer, L. F. Lopez, J. Bohlen, L. Richter, F. Cole, P. Tinnefeld, F. D. Stefani

“Pulsed Interleaved MINFLUX”

Nano Letters 21 (2021) 840-846

L. A. Masullo, L. F. Lopez, F. D. Stefani

“A common framework for single-molecule localization using sequential structured illumination”

Biophysical Reports 2 (2022) 100036

L. A. Masullo, F. D. Stefani

“Multiphoton single-molecule localization by sequential excitation with light minima”

Light: Science & Applications 11 (2022) 70

L. A. Masullo, A. M. Szalai, L. F. Lopez, M. Pilo-Pais, G. P. Acuna, F. D. Stefani

“An alternative to MINFLUX that enables nanometre resolution in a confocal microscope”

Light: Science & Applications  (2022) – just accepted

DJEGHDI Kenza ,  AMI, Université de Fribourg

3D tomographic analysis of diamond-based structures of varying order in Pachyrhynchus weevils 

Numerous organisms rely on structural colour to provide crucial survival functions, such as camouflage and sexual signalling. The bright colours of Pachyrhynchus weevils, for example, originate mainly from complex, dielectric, periodic nano-structures present within their elytral scales.  Previous work has shown that this structure is exclusively an ordered single-network diamond-type photonic crystal. We here investigate the Pachyrhynchus congestus mirabilis, an Easter egg weevil with red and blue spots on its elytra. This species stands out as its scales feature either a crystalline diamond structure (leading to red colour) or an amorphous diamond-based structure (blue colour). While so far studies of such 3D photonic structures have heavily relied on 2D data collection, we collected 3D datasets on both types of structures by FIB-tomography using a novel in-situ Pt-based backfilling procedure. Compared to a resin-based filling, this procedure allows precise control over the depth and area of deposition, enhancing contrast while reducing most detrimental effects, i.e. curtaining or charge accumulation. The sub-20 nm resolution and fidelity of the datasets allow for precise reconstruction of sizable volumes and enables a quantitative in-depth structural characterisation that revealed hidden correlations in the amorphous morphology. Based on optical measurements and full-wave simulations, the colouration of both scales is caused by a partial bandgap opening. This study sheds light on the structural and optical properties of both quasi-ordered networks in “blue scales” and their highly ordered crystalline counterparts in “red scales”, but also gives hints on the developmental pathway that leads to their formation.

Jeana Zheng , Physics, New York University, Physics 

Diffusion of DNA-coated colloids on DNA coated surface

DNA-coated colloids can self-assemble and crystalize into a wide variety of structures. In order for DNA-coated colloids to anneal and form crystals, they must roll and diffuse while attached to each other.  Here we report on the diffusion of DNA-coated colloidal spheres on a flat DNA-coated substrate.  Near the DNA-melting temperature, the mean square displacement is linear in time as expected for normal diffusion, but the diffusion coefficient is much smaller than for free diffusion. As the temperature is lowered, the motion becomes sub-diffusive, which suggests the presence of random free energy barriers in the DNA-mediated interactions. We have found that DNA induced interactions are highly sensitive to the density and homogeneity of the DNA distribution. As we reduce the DNA density, the DNA coated colloids diffuse slower. This study is important for designing and optimizing self-assembly structure of DNA coated colloids.

Funding: MRSEC Program of the National Science Foundation under Award Number DMR-1420073 

Vinod Kumar Saranathan, Division of Science, Yale-NUS College, Singapore, Department of Biological Sciences, National University of Singapore 

Evolutionary Photonics: Structure, Function, Development and Biomimetics of Self-assembled Organismal Photonic Nanostructures

Colors in Nature can be produced either chemically, by the selective light absorption by pigments, or physically, by light interference from biophotonic nanostructures. Intriguingly, there are almost no known violet, blue or green pigments in animals. And yet these structurally produced colors are ubiquitous in nature and constitute an important aspect of the overall appearance of organisms, as they are frequently used in camouflage, and in social and sexual communication. As the underlying biophotonic nanostructures are overwhelmingly diverse in form and function, their structural and optical characterization has hitherto remained challenging despite centuries of research, which is where I have made rapid and significant contributions. Although there is a burgeoning interest on structural colors from biologists, physicists and engineers, we currently lack an explicit comparative framework, which is essential to understand how these biological signals function, and evolve in organisms. Moreover, the mechanisms controlling the morphogenesis of these complex, biologically patterned nanostructures are much too large to be described by conventional cell or molecular biology, and much too small to be captured by traditional developmental biology. As a consequence, we know very little about the development of photonic nanostructures within cells, beyond the realisation that they are self-assembled intra-cellularly by mechanobiological, phase separation and micro-phase separation like processes. Biophotonic nanostructures are also of broader interest to materials science and engineering, since the facile synthetic fabrication of three-dimensional photonic nanostructures at these rather large optical length scales (200-500 nm) is challenging. Organismal structural colors that have evolved over millions of years to function in a variety of signalling contexts are an ideal source to look for naturally optimized solutions to technological problems in sensing, photonics, etc. In this talk, I will summarise our current knowledge about the structure, function and morphogenesis of biophotonic nanostructures and how this can be leveraged for the biomimetic or bio-inspired synthesis of next generation photonic meta-materials and devices.

Roel Dullens, Department of Chemistry, Physical and Theoretical Chemistry Laboratory, University of Oxford, South Park Road, OX1 3QZ, Oxford, United Kingdom 

Shaping colloidal SU-8 polymer particles: from rods to spheres to bananas

Colloidal dispersions of rod-like particles are widely accepted as convenient model systems to study the phase behavior of liquid-crystal forming systems, commonly found in LCDs. This is due to the fact that colloidal rods exhibit analogous phase behavior to that of elongated molecules, while they can be directly observed by optical microscopy. More recently, there has also been a burst of interest in the liquid crystalline behaviour of so-called bent-core, or banana-shaped, molecules, which have been predicted to form exotic biaxial nematic phases such as the twist-bend and splay-bent nematic phase. These may be of particular interest due to their fast switching response in LCDs. While there have been claims of the observation of these biaxial nematic phases in molecular bent-core systems, no colloidal bananas have been reported in which these the structure and dynamics of these phases could be imaged at the particle level.

Here, we have developed an entirely new family of colloidal SU-8 polymer particles, with shapes ranging from rods, sphero-cylinders, spheres and bananas with tuneable length, diameter and curvature that are stable in both aqueous and apolar solvent mixtures. Our colloidal SU-8 polymer particles are produced in bulk and by varying the composition of the solvent mixture, both the difference in refractive index and mass density between the particles and the solvent can be independently controlled. This, for example, enables the use of colloidal SU-8 rods in both 3D confocal microscopy and optical trapping experiments, and even in experiments combining both techniques, while the effect of gravity can be carefully tuned. Particularly exciting is that we can tune the curvature of the bananas, which is a key parameter in their phase behaviour. This is demonstrated by our observations of the exotic structures formed by differently curved bananas, including isotropic, anti-ferromagnetic smectic and even the elusive splay-bend nematic ordering and a completely new `colloidal vortex liquid'. 

Jakub Haberko, Faculty of Physics and Applied Computer Science, AGH University of Science and Technology, Krakow, Poland

Metamaterials for controlling light polarization and asymmetric transmission of light 

Controlling the polarization state of light is of utmost importance in many scientific and industrial applications, such as stereoscopic vision, polarization microscopy or non-linear light frequency conversion. Typically, to achieve this goal dichroism, birefringence or optical activity are utilized. However, designer metamaterials, both metallic and dielectric, can also be applied. Recently novel nano/microfabrication techniques, such as two-photon laser nanolithography, have allowed such structures to be realiized. On the other hand computer simulation techniques facilitate the search for innovative metamaterial designs. I will present two different concepts: i) a successfully designed and fabricated array of twisted bands and ii) a metasurface consisting of quasi-random metallic pixels, designed with a stochastic search algorithm, acting as a waveplate in the near- to mid-infrared range.

Another interesting group of materials are those which exhibits a significant difference in transmission for light propagating with opposite wave vectors. I will show two designs based on a similar principle, where light passes through an all-dielectric structure consisting of a Bragg mirror and a grid of periodic lines or pillars. In both cases considerable transmission asymmetry has been achieved in the NIR regime with one of the designs exhibiting polarizationindependent performance. 

Maxime J. Bergman, Department of Chemistry, Lund University, Sweden

Investigation of the interaction potential of soft, thermoresponsive microgels

Thermosensitive microgels are soft colloids that have gained popularity in recent years. They display a rich phase behaviour due to their unique soft potential and internal structure. Especially, their thermosensitivity allows us to use temperature as an external control to tune particle size, volume fraction and effective interaction potential in situ. It is relatively easy to incorporate acrylic acid during synthesis, giving rise to ionic microgels which are dressed with additional sensitivity to (amongst others) pH and salinity. 

However, a quantitative understanding of the effective interactions between microgels is still lacking. Previous efforts have shown progress, but an all encompassing interaction potential predicting the full phase behaviour for these colloids has not been found yet. In the case of ionic microgels, an interaction potential – balancing the polymeric and charged nature of the colloid – has been proposed but never thoroughly tested.

In this seminar, I will discuss our recent efforts in pinpointing the interaction potential of both neutral and ionic microgels. Our main approach is through quantitative comparison of simulated pair correlation functions (g(r)s) and experimentally obtained g(r)s. In the case of neutral microgels, we find that a (multi)-Hertzian potential works well to describe both structure and dynamics of the probed state points, and that a core repulsion emerges once microgels are forced together. For the ionic microgels, their undefined swelling response to salinity and pH is investigated first via static and dynamic light scattering. Based on the surprising microgel architectures found under various charging conditions, we consider a new model to describe their interactions.

Martin E. Leser , Nestlé Research Center Lausanne, Institute of Materials Science, Colloidal Systems group, Vers-chez-les Blanc, 1000 Lausanne 26, Switzerland 

Food Material Science – The Key to the Design of new Functionality in Food Products    

The food industry is constantly challenged to meet consumer demands for new food products that are safe, convenient, affordable, pleasurable and healthy. Especially the increasing importance of health & wellbeing from a healthcare system perspective urges food developers to accelerate their research and development work to come up with new innovative and sustainable products. Thereby, formulation science and process design play a major role. However, the impact on food structure is in many development activities today still poorly addressed and/or understood. 

In order to develop and produce new tasty, nutritious and healthy food products the creation of new know-how on how to better tailor, design or modulate colloidal food structures, their interfaces and their interactions is required. Although foods are complex systems, it becomes more and more evident that applying ‘Soft Condensed Matter’ Physics concepts allows to better understand structure formation and dynamics in food materials. The main principle of applying the ‘Soft Matter approach’ in designing new food materials lays in the better understanding of the formation of ‘mesoscopic structures’ of different length scales and different dynamics. 

In my talk I will first give a short introduction into main consumer trends and show how they changed over the last 2 decades or so. Then I will illustrate how the food industry is trying to react on these trends, and illustrate how the changes in trends necessitate also an adaptation of the needed research efforts, activities and focus. 

Sergey SKIPETROV, Laboratoire de Physique et Modelisation des Milieux Condenses  

Anderson localization of vector waves

Anderson localization was first discovered for electrons in disordered solids but later was shown to take place for various types of waves in disordered media. For three-dimensional (3D) disorder, it takes place only in a restricted band of frequencies, separated from the rest of the spectrum by mobility edges, and  only when the disorder is strong enough. Our recent results indicate that the vector nature of waves (microwaves, light, elastic waves) used in the experiments on Anderson localization, plays an important role. In particular, the transverse electromagnetic waves cannot be localized by a random 3D arrangement of resonant point-like scatterers (atoms), whereas the elastic waves, which have a longitudinal component as well, can be localized in a way very similar to scalar waves. However, the localization of light can still be made possible by putting the atoms in a strong external magnetic field. We will present a unified view on Anderson localization and compute the localization phase diagrams and the critical parameters (mobility edges and critical exponents) of Anderson localization transitions for elastic waves and light scattered by atoms in a strong magnetic field. Despite the differences between these two systems, they turn out to belong to the same universality class. 

François A. Lavergne, Department of Physics, University of Konstanz, Germany

Grain boundary loops in 2D colloidal crystals: formation, kinetics, and dislocation structure

Understanding the physics of grain boundaries is essential to tune the properties of polycrystalline materials. In 2D materials, a grain boundary merely consists of a line of topological defects, called dislocations. Linking the formation and dynamics of grain boundaries to their dislocation structure is challenging since they form a complex network that evolves during grain growth. A simpler case to consider is that of a grain boundary loop, for which the line of dislocations closes onto itself. The loop then encloses a crystalline patch rotated by some angle with respect to the surrounding crystal, and its dynamics is purely driven by interfacial curvature, while preserving an overall zero “topological charge”.

Here, we use an optical vortex to rotate a circular patch of colloidal crystal, and monitor the recovery to the deformation after the vortex is turned off using optical microscopy. We find that if the product of the radius of the patch and the angle of rotation exceeds a critical value, a grain boundary loop is formed. This plastic deformation is restored by spontaneous shrinkage of the grain boundary loop, at a rate that solely depends on this product, despite complex pathways of dislocation reactions. Below the critical value however, the deformation is elastically restored. We show that the onset of plasticity under this rotational deformation results from the unique dislocation structure of grain boundary loops. As a consequence, the critical value is general in hexagonal 2D lattices, and corresponds to a dislocation structure equivalent to the “flower defect” in graphene. Strikingly, the critical value can be directly measured from the divergence of the shrinkage rate, which is a clear dynamic signature of the onset of plasticity in the case of a colloidal crystal.

Laurence Ramos, Laboratoire Charles Coulomb, University of Montpellier, France

Microscopic dynamics during failure of soft materials

Fatigue and material failure are ubiquitous, with implications from geology to everyday life and material science. A deeper understanding of the microscopic mechanisms eventually leading to fracture or yielding is clearly required. In this optics, we have built an original setup coupling light scattering and rheology, which allow simultaneous measurements of the macroscopic deformation and the microscopic dynamics of soft materials, while applying a shear stress. I will first show that the microscopic dynamic of a colloidal gel, a model network-forming system, exhibits dramatic changes, when submitted to a constant stress. The network macroscopic failure is preceded by qualitative and quantitative changes of the dynamics, from reversible particle displacements to a burst of irreversible plastic rearrangements. 

I will then present results on the fatigue of a soft glass submitted to repeated oscillatory shear deformations. By tracking both the reversible non affine dynamics and the decay of higher order correlation echoes, several dynamical regimes are found when the shear amplitude varies. In particular, we find over a relatively broad range of strain amplitude, which macroscopically corresponds to the regime of a prominent loss peak in the rheology data, the coexistence of a fast liquid-like mode and a slower solid-like mode.

Helmut Ritsch, University of Innsbruck, Austria  

Attractive force on and in between atoms due to blackbody radiation  

Electromagnetic radiation fields carry energy and momentum and thus induce forces when scattered from material objects. These forces are very well studied and understood for monochromatic laser fields and form the physical basis of opto-mechanics, optical tweezers and laser cooling. Theory exhibits a clear distinction between radiation pressure from resonant absorption and spontaneous emission and gradient or dipole forces from off resonant coherent scattering. Surprisingly both components are also significantly present in incoherent broad band radiation fields and even for blackbody radiation. In contrast to naïve expectations the radially purely outward directed radiation energy flow from a hot sphere still can creates an attractive force on nearby atoms due to the dominance of the attractive dipole force over repulsive radiation pressure. This renders a hot sphere into a radiative dipole trap for atomic hydrogen up to a temperature of about 6000K. While this force is small it still dominates gravity up to certain mass ratios. 

Interestingly, despite their minute size at room temperature, these forces create important perturbations in current state of the art atom interferometers and optical atomic clocks. In a recent experiment at Berkeley using atom interferometry the attractive force induced by blackbody radiation between a cesium atom and a heated, centimeter-sized cylinder was determined to be orders of magnitude stronger than the outward directed radiation pressure. The observed force dominates over both gravity and radiation pressure, and does so for a large temperature range. 

When few particles are in a close vicinity, their scattered fields interfere and one gets a collective modification of light forces with particularly strong inter particle forces. This collective effect survives in a modified form in broadband fields leading to extra near field attraction and sometimes far field repulsion, leading to instabilities and self-ordering and spectral radiation redistribution in illuminated cold clouds. 

[1] Sonnleitner, Matthias, Monika Ritsch-Marte, and Helmut Ritsch. "Attractive optical forces from blackbody radiation." Physical review letters 111.2 (2013) 

[2] Haslinger, Philipp, et al. "Attractive force on atoms due to blackbody radiation." Nature Physics (2017)

[3] Holzmann, Daniela, and Helmut Ritsch. "Tailored long range forces on polarizable particles by collective scattering of broadband radiation." New Journal of Physics 18.10 (2016): 103041. 

Davide Pierleoni, Department of Civil, Chemical, Environmental and Materials Engineering, University of Bologna

Dynamic scanning gravimetric techniques: solvent-induced glass transition measurements by novel methods

An innovative experimental technique was proposed as novel characterization method to evaluate both glass transition and glassy polymers relaxation behavior as induced by solvents, resembling calorimetric or volumetric tests.

Several test approaches were attempted to provide evidences of reliable and flexible methods for observation of glassy dynamics. These methods consist of simple gravimetric experiments in which solvent atmosphere is dynamically controlled. A well-known polymer-plasticizer system as polystyrene-toluene was here focused for demonstration purpose only; nevertheless, the novel approach allowed the identification of interesting phenomena, e.g. retrograde vitrification, besides other relevant polymer-solvent couple properties as glass transition solvent content.

High reliability and reduction in experimental times were the most significant features of proposed method with respect to classic isothermal methods used for glassy dynamics induced by plasticizers. But in addition to these unquestionable advantages, the method allowed to easily extend the experimental analysis to additional conditions that were not documented before, as transitions obtained in sorption tests under isobaric (namely the penetrant partial pressure) and iso-activity environments: these were adopted as alternatives to control the solvent content into the polymer specimen under investigation.

The analysis of obtained results allowed for the evaluation of a glass transition region in each experimental condition tested, in terms of solvent pressure or temperature values. Therefore, the novel technique discloses new measurement opportunities not easily accessible with traditional techniques, combined with a remarkable versatility of the experimental apparatus.

Appropriate modeling analysis of the results obtained by differential tests on pure polymers can be able to retrieve important structural information on polymer glass/melt relaxation, as it was widely addressed on literature. To this aim, a relaxation time scaling approach based on Grassia and D’Amore model was applied to evaluate solvent solubility, when a well-controlled previous history on the polymer glass was ensured.