Nicolas Dorsaz

Arrested demixing opens route to bigels

Understanding and, ultimately, controlling the properties of amorphous materials is one of the key goals of material science. Among the different amorphous structures, a very important role is played by colloidal gels. It has been only recently understood that colloidal gels are the result of the interplay between phase separation and arrest. When short-ranged attractive colloids are quenched into the phase-separating region, density fluctuations are arrested and this results in ramified amorphous space-spanning structures that are capable of sustaining mechanical stress. We present a mechanism of aggregation through arrested demixing in binary colloidal mixtures, which leads to the formation of a yet unexplored class of materials––bigels. This material is obtained by tuning interspecies interactions. Using a computer model, we investigate the phase behavior and the structural properties of these bigels. We show the topological similarities and the geometrical differences between these binary, interpenetrating, arrested structures and their well-known monodisperse counterparts, colloidal gels. Our findings are supported by confocal microscopy experiments performed on mixtures of DNA-coated colloids. The mechanism of bigel formation is a generalization of arrested phase separation and is therefore universal.

New insight into cataract formation : Enhanced stability through mutual attraction

Small-angle neutron scattering experiments and molecular dynamics simulations combined with an application of concepts from soft matter physics to complex protein mixtures provide new insight into the stability of eye lens protein mixtures. Exploring this colloid-protein analogy we demonstrate that weak attractions between unlike proteins help to maintain lens transparency in an extremely sensitive and nonmonotonic manner. These results not only represent an important step towards a better understanding of protein condensation diseases such as cataract formation, but provide general guidelines for tuning the stability of colloid mixtures, a topic relevant for soft matter physics and industrial applications.

Colloidal characterization and thermodynamic stability of binary eye lens protein mixtures.

We present a study of binary mixtures of eye lens crystallin proteins. A coarse-grained model of aqueous alpha- and gamma-crystallin mixtures based on molecular dynamics simulations and SANS experiments is proposed. Thermodynamic perturbation theory is implemented to obtain the stability boundaries, or spinodal surface, of the binary mixture in the full parameter space. The stability of these high-concentration crystallin mixtures was found to depend on the alpha-gamma attraction in a manner that is both extremely sensitive and nonmonotonic; stronger or weaker attraction resulted in a spectacularly enhanced instability. The relevance of these mechanisms as possible sources of the alteration of the spatial distribution of the lens proteins encountered in cataract disease is discussed.

Hard sphere-like glass transition in eye lens α-crystallin solutions.

Significance Normal vision and accommodation rely on the clarity and softness of the eye lens. Hardening of the lens has been linked with presbyopia, the loss of accommodative capability with age, and lens clarity is disrupted in cataract, a leading cause of blindness worldwide. Here, realistically concentrated solutions of a prevalent eye lens structural protein, α-crystallin, which exhibits short-range order needed for lens transparency, are found in addition to show high-concentration dynamical slowing down similar to that of hard-sphere glass transitions. This suggests that analogous investigation of concentrated crystallin mixtures, like those in the living lens, may help to advance understanding of the molecular basis of both presbyopia and cataract. We study the equilibrium liquid structure and dynamics of dilute and concentrated bovine eye lens α-crystallin solutions, using small-angle X-ray scattering, static and dynamic light scattering, viscometry, molecular dynamics simulations, and mode-coupling theory. We find that a polydisperse Percus–Yevick hard-sphere liquid-structure model accurately reproduces both static light scattering data and small-angle X-ray scattering liquid structure data from α-crystallin solutions over an extended range of protein concentrations up to 290 mg/mL or 49% vol fraction and up to ca. 330 mg/mL for static light scattering. The measured dynamic light scattering and viscosity properties are also consistent with those of hard-sphere colloids and show power laws characteristic of an approach toward a glass transition at α-crystallin volume fractions near 58%. Dynamic light scattering at a volume fraction beyond the glass transition indicates formation of an arrested state. We further perform event-driven molecular dynamics simulations of polydisperse hard-sphere systems and use mode-coupling theory to compare the measured dynamic power laws with those of hard-sphere models. The static and dynamic data, simulations, and analysis show that aqueous eye lens α-crystallin solutions exhibit a glass transition at high concentrations that is similar to those found in hard-sphere colloidal systems. The α-crystallin glass transition could have implications for the molecular basis of presbyopia and the kinetics of molecular change during cataractogenesis.

Spiers Memorial Lecture: Effect of interaction specificity on the phase behaviour of patchy particles

We report a numerical study on the phase behaviour of a ‘patch–anti-patch’ model for particles with tetrahedrally arranged attractive spots. In particular, we compute the phase equilibria between the fluid and a low density diamond cubic (DC) crystal for different realizations of the patch–anti-patch interaction. By increasing the ‘specificity’ of the patches, i.e. lowering the number of corresponding attractive ‘anti-patches’ to a given patch, we find that the metastability gap between the DC freezing boundary and the liquid–gas critical point widens considerably. We argue that this effect of interaction specificity is relevant for the description of protein phase diagrams, as patch–anti-patch interactions can stabilise relatively open, ordered structures.

Scaling laws for jets of single cavitation bubbles

Fast liquid jets, called micro-jets, are produced within cavitation bubbles experiencing an aspherical collapse. Here we review micro-jets of different origins, scales and appearances, and propose a unified framework to describe their dynamics by using an anisotropy parameter zeta >= 0, representing a dimensionless measure of the liquid momentum at the collapse point (Kelvin impulse). This parameter is rigorously defined for various jet drivers, including gravity and nearby boundaries. Combining theoretical considerations with hundreds of high-speed visualisations of bubbles collapsing near a rigid surface, near a free surface or in variable gravity, we classify the jets into three distinct regimes: weak, intermediate and strong. Weak jets (zeta 0.1) pierce the bubble early during the collapse. The dynamics of the jets is analysed through key observables, such as the jet impact time, jet speed, bubble displacement, bubble volume at jet impact and vapour-jet volume. We find that, upon normalising these observables to dimensionless jet parameters, they all reduce to straightforward functions of zeta, which we can reproduce numerically using potential flow theory. An interesting consequence of this result is that a measurement of a single observable, such as the bubble displacement, suffices to estimate any other parameter, such as the jet speed. Remarkably, the dimensionless parameters of intermediate and weak jets (zeta < 0.1) depend only on zeta, not on the jet driver (i.e. gravity or boundaries). In the same regime, the jet parameters are found to be well approximated by power laws of zeta, which we explain through analytical arguments.

Confined shocks inside isolated liquid volumes: A new path of erosion?

The unique confinement of shock waves inside isolated liquid volumes amplifies the density of shock-liquid interactions. We investigate this universal principle through an interdisciplinary study of shock-induced cavitation inside liquid volumes, isolated in 2 and 3 dimensions. By combining high-speed visualizations of ideal water drops realized in microgravity with smoothed particle simulations we evidence strong shock-induced cavitation at the focus of the confined shocks. We extend this analysis to ground-observations of jets and drops using an analytic model, and argue that cavitation caused by trapped shocks offers a distinct mechanism of erosion in high-speed impacts (>100 m/s).

Inertial effects in diffusion-limited reactions

Diffusion-limited reactions are commonly found in biochemical processes such as enzyme catalysis, colloid and protein aggregation and binding between different macromolecules in cells. Usually, such reactions are modeled within the Smoluchowski framework by considering purely diffusive boundary problems. However, inertial effects are not always negligible in real biological or physical media on typical observation time frames. This is all the more so for non-bulk phenomena involving physical boundaries, that introduce additional time and space constraints. In this paper, we present and test a novel numerical scheme, based on event-driven Brownian dynamics, that allows us to explore a wide range of velocity relaxation times, from the purely diffusive case to the underdamped regime. We show that our algorithm perfectly reproduces the solution of the Fokker-Planck problem with absorbing boundary conditions in all the regimes considered and is thus a good tool for studying diffusion-guided reactions in complex biological environments.

Energy partition at the collapse of spherical cavitation bubbles.

Spherically collapsing cavitation bubbles produce a shock wave followed by a rebound bubble. Here we present a systematic investigation of the energy partition between the rebound and the shock. Highly spherical cavitation bubbles are produced in microgravity, which suppresses the buoyant pressure gradient that otherwise deteriorates the sphericity of the bubbles. We measure the radius of the rebound bubble and estimate the shock energy as a function of the initial bubble radius (2-5.6mm) and the liquid pressure (10-80kPa). Those measurements uncover a systematic pressure dependence of the energy partition between rebound and shock. We demonstrate that these observations agree with a physical model relying on a first-order approximation of the liquid compressibility and an adiabatic treatment of the noncondensable gas inside the bubble. Using this model we find that the energy partition between rebound and shock is dictated by a single nondimensional parameter ξ=Δpγ6/[p(g0)1/γ(ρc2)1-1/γ], where Δp=p∞ - pv is the driving pressure, p∞ is the static pressure in the liquid, pv is the vapor pressure, pg0 is the pressure of the noncondensable gas at the maximal bubble radius, γ is the adiabatic index of the noncondensable gas, ρ is the liquid density, and c is the speed of sound in the liquid.

Diffusion-limited reactions in crowded environments: a local density approximation

In the real world, diffusion-limited reactions in chemistry and biology mostly occur in crowded environments, such as macromolecular complex formation in the cell. Despite the paramount importance of such phenomena, theoretical approaches still mainly rely on the Smoluchowski theory, only valid in the infinite dilution limit. In this paper we introduce a novel theoretical framework to describe the encounter rate and the stationary density profiles for encounters between an immobilized target and a fluid of interacting spherical particles, valid in the local density approximation. A comparison with numerical simulations performed for a fluid of hard spheres and square well attractive hard spheres confirms the accuracy of our treatment.