Physics of virus self-assembly and virus-like biological nanoparticles

Physics of Virus Self-Assembly

Surface activity and micelle formation of alkylguanidinium chlorides, containing 10, 12, 14, and 16 carbon atoms in the hydrophobic tail have been studied by combining conductivity and surface tension measurements with isothermal titration calorimetry. The purity of the resulting surfactants, their temperatures of Cr → LC and LC → I transitions, as well as their propensity of forming birefringent phases were assessed based on the results of 1H and 13C NMR, differential scanning calorimetry (DSC), and polarizing microscopy studies. Whenever possible, the resulting values of Krafft temperature (TK), critical micelle concentration (CMC), minimum surface tension above the CMC, chloride counter-ion binding to the micelle, and the standard enthalpy of micelle formation per mole of surfactant (micH°), were compared to those characterizing alkyltrimethylammonium chlorides or bromides with the same tail lengths. The value of TK ranged between 292 and 314 K and increased strongly with increase in the chain length of the hydrophobic tail. Micellization was described as both entropy and enthalpy-driven. Based on the direct calorimetry measurements, the general trends in the CMC with the temperature, hydrophobic tail length and NaCl addition were found to be similar to those of other types of cationic surfactants. The particularly exothermic character of micellization was ascribed to the hydrogen-binding capacity of the guanidinium head-group.

Material failure is ubiquitous on length scales ranging from a few nanometers, as in fracture of atomic or molecular systems, up to geological scales, as in earthquakes. The detection of any precursors that may point to incipient failure is the Holy Grail in many disciplines, from material science to engineering and geology. Material failure is particularly relevant to soft matter systems (colloids, emulsions, polymers etc.), which are an ideal benchmark to investigate how mechanical stress impacts condensed matter. Indeed, soft matter is very susceptible to even modest mechanical loads and most soft materials can be conveniently investigated by powerful optical methods such as microscopy or light scattering.We have built up a shear-cell that allows us to couple rheology to light scattering measurements on soft solids. In this talk I will focus on a fractal colloidal gel produced by in-situ aggregation of silica nanoparticles in water and show how reversible and irreversible (microscopic and macroscopic) deformations, build up when the gel is submitted to a shear stress. Thanks to our set-up, we are able to elucidate the physical processes at play at the onset of yielding.

Designing stable liquid crystal (LC) composites is one of the main tasks pursued by several members of theIC1208 Cost action. Liquid crystal composites are however colloidal dispersions which have very specificproperties compared to the usual colloidal dispersions in simple liquids. LC matrices give indeed rise to longrangeattractive multipolar interactions between colloidal dispersions [1]. Strong dipolar interactions are mainlyobserved at large scale, but even at the nanometer scale the interactions between two nanoparticles arequadrupolar, weaker but often sufficient to yield aggregates in many systems [1]. Aggregation phenomena undersuch multipolar interactions are still not fully understood, so to get a deeper understanding we have considered a2D model system describing the classical dynamics of elongated particles at a liquid interface.
When elongated particles are trapped at a liquid interface they distort it and then interact via quadrupolarcapillary interactions. Furthermore the interactions between a group of already aggregated particles and a singleone located at a large distance strongly depend on the spatial arrangement of the aggregated particles. Thisphenomenon is further complicated by the presence of possible direct solid-solid interactions, which would arisewhen the particles enter into contact and possibly freeze the aggregate shapes. Currently we are focusing onmodeling the above described aggregation phenomena by developing a numerical code that is sufficientlyaccurate in order to catch the many-body interactions mentioned above in a simple way. Once this is achievedextensive comparison with experiments will be possible and the model system may be used to address openquestions arising from the experiments. Our model is based on the 2D solution of the Young-Laplace equation togain the forces acting on the particles, then moving each particle individually by solving the Newton equationsbased on classical discreet element methods. In this presentation I would like to give an insight into the currentstate of the development of the numerical model and our future objectives.
[1] Blanc, C., Coursault, D., Lacaze, E., Ordering nano-and microparticles assemblies with liquid crystals, Liquid Crystals Reviews, 1(2),pp. 83-109, (2013).12

Liquid crystal (LC) colloidal dispersions have been shown to promote complex ordered organizations due to the local distortion of the host by the particles and the resulting long-range elastic interactions mediated by the Frank elasticity. The key components responsible for these colloidal patterns have been well understood at the micron scale and are now increasingly studied at the sub-micron scale. One of the main challenges in order to design useful colloidal dispersions in a nematic however remains the fine control of the multipolar elastic interactions. The attractive components are indeed usually sufficiently strong to lead to a rapid aggregation of dispersed particles. We will first discuss some strategies to prevent this phenomenon in bulk.
We will also discuss the case of particles trapped at liquid crystal interfaces where the elastic interactions are strongly modified by the confinement [1,2] but also might compete with capillary interactions, as we detailed recently [3]. When confined at LC interfaces, spherical particles with homeotropic anchoring interact with repulsive interactions only, then forming regular arrays with large lattice parameters. When they are confined in a thin nematic film the colloidal behaviour is much more complex depending on the boundary conditions at the interfaces. For example we have shown that the capillary attraction expected for particles confined in thin films with hybrid anchorings is counteracted by the nematic texture and the presence of topological defects.
[1] M.A. Gharbi, M. Nobili, M. In, G. Prévot, P. Galatola, J.-B. Fournier, and Ch. Blanc, "Behavior of colloidal particles at an air/nematic liquid crystal interface", Soft Matter, 7, 1467-1471, (2011).
[2] M. A. Gharbi, M. Nobili, Ch. Blanc, "Use of topological defects as templates to direct assembly of colloidal particles at nematic interfaces",J. Colloid. Interface Science, 417 ,250 (2014).
[3] Jeridi Haifa, Gharbi M. A., Othman Tahar, and Blanc C., "Capillary-induced giant elastic dipoles in thin nematic films", PNAS, 112 , 14771–14776 (2015).

We report on the unusual and counterintuitive behavior of spin lifetime of excitons in coupled semiconductor quantum wells (CQWs) in the presence of in-plane magnetic field. Instead of conventional acceleration of spin relaxation due to the Larmor precession of electron and hole spins, we observe a strong increase of the spin relaxation time at low magnetic fields followed by saturation and decrease at higher fields. We argue that this nonmonotonic spin relaxation dynamics is a fingerprint of the magnetic quantum confined Stark effect. In the presence of electric field along the CQW growth axis, an applied magnetic field efficiently suppresses the exciton spin coherence, due to inhomogeneous broadening of the g-factor distribution.