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Scientific highlights 2012

Orientational Order of Carbon Nanotube Guests in a Nematic Host Suspension of Colloidal Viral Rods

N.Puech, M.Dennison, Ch.Blanc, P.van der Schoot, M.Dijkstra, R.van Roij, P.Poulin and E.Grelet
SoftComp partner: CNRS Bordeaux, CNRS Montpellier, Univ. Utrecht
PRL 108 (2012) 247801

In order to investigate the coupling between the degrees of alignment of elongated particles in binary nematic dispersions, surfactant stabilized single-wall carbon nanotubes (CNTs) have been added to nematic suspensions of colloidal rodlike viruses in aqueous solution. We have independently measured the orientational order parameter of both components of the guest-host system by means of polarized Raman spectroscopy and by optical birefringence, respectively. Our system allows us therefore to probe the regime where the guest particles (CNTs) are shorter and thinner than the fd virus host particles. We show that the degree of order of the CNTs is systematically smaller than that of the fd virus particles for the whole nematic range. These measurements are in good agreement with predictions of an Onsager-type second-viral theory, which explicitly includes the flexibility of the virus particles, and the polydispersity of the CNTs.
The present result offers a unique route towards the development of composites that combine a high degree of alignment of the matrix components while preserving some disorder of the smaller inclusions. Such a combination is of great technological interest for composites that are expected to exhibit good mechanical properties arising from the alignment of the matrix components, and electrical or thermal conductivity arising from the formation of percolated networks of the inclusions.

Measured (symbols) and calculated (lines) orientational order parameters of the host nematic phase of fd virus suspensions Sfd (black squares) doped with surfactant-stabilized single wall carbon nanotubes SCNT (red circles) as a function of the virus concentration cfd.Theoretical results show the dependence of SCNT for different CNT lengths, LCNT.

Hydrophobic Interactions Modulate Self-Assembly of Nanoparticles

A.Sánchez-Iglesias, M.Grzelczak, T.Altantzis, B.Goris, J.Perez-Juste, S.Bals, G.Van Tendeloo, S.H.Donaldson Jr., B.F.Chmelka, J.N.Israelachvili and L.M.Liz-Marzán
SoftComp partner: Universidade de Vigo (Group Prof. Liz-Marzán), Spain
ACS Nano, 2012, 6, 11059-11065

In this article, the solvent-induced reversible self-assembly of gold nanoparticles into nanoclusters with controllable sizes is demonstrated to be dominated by hydrophobic interactions. It is shown that polystyrene (PS)-stabilized spherical gold nanoparticles dispersed in tetrahydrofuran can form clusters of up to 904 nm in diameter upon addition of water, a poor solvent for PS. Theoretical examinations suggest that the main attractive force forming the clusters is hydrophobicity. By adding a stabilizing amphiphilic diblock copolymer, geometrical features of the nanoparticle clusters, such as aggregate size and interparticle distance can be controlled. Such control over cluster size, interparticle distance, and overall optical response, makes these structures promising candidates for drug delivery, especially if release of internal cargo is required. It is important to note, that the copolymer used here is a model system that can be replaced by conducting or biodegradable copolymers bringing new solutions for problems in biosensing or energy conversion.

The graph shows the effect of hydrophobic interactions on the colloidal stability of gold nanoparticles. Colloid-colloid interaction energy E as a function of interparticle separation D. Up: THF solution, down: Colloidal stable in THF-water mixture.

Helix formation of semiflexible polymers in flow through structured microchannels

R. Chelakkot, R.G. Winkler and G. Gompper
SoftComp partner: Forschungszentrum Juelich (Group Prof. Gompper), Germany
Physical Review Letters, 2012, 109, 178101 [1-5]

Buckling is a common phenomena of slender bodies, like long filaments and thin sheets, under an external load. Typically, buckling transitions are considered under equilibrium conditions. The transport of semiflexible polymers in microchannels and capillaries presents a new opportunity to study the non equilibrium behavior of such filaments. Vice versa, a detailed understanding of the dynamical processes involved in such a transport is of paramount importance in many applications, in particular for the flow behavior of many biologically-relevant polymers, such as DNA, actin filaments, and microtubules. Non-equilibrium instabilities can appear under various conditions. We investigate flow fields of spatially varying flow strength. Such a situation is easily realized in flows through spatially-structured microchannels. By mesoscale hydrodynamic simulations, we observe the buckling of a semiflexible polymer as it enters a wider channel section. Buckling is often the first step in the formation of more complex structures. Indeed, we observe a subsequent flow-induced helical coiling of the polymer. The helix properties are studied in their dependence on the diameter ratio of the channel, the polymer bending rigidity, and the flow strength.

Polymer conformation in channels with the diameter ratio of wide and narrow parts Dw/Dn=2. The flow strength is characterised by the Peclet number Pe=550. The polymer length equals the persistence length.

Fluctuating shells under pressure

J.Paulose, G.A.Vliegenthart, G.Gompper and D.R.Nelson
SoftComp partner: Forschungszentrum Juelich (Group Prof. Gompper), Germany
Proc. Natl. Acad. Sci. USA 109 (2012) 19551

A thin spherical shell can be considered as an elastic membrane with shear modulus, bending rigidity and a non-zero curvature. On a micrometer and submicrometer scale, examples of these shells range from synthetic hollow polyelectrolyte capsules with important technological applications to biologically relevant systems such as red blood cells and spherical virus capsids. In the absence of thermal fluctuations the stability of elastic shells against external forces such as a uniform pressure field or a point like indentation depends on the ratio between size of the shell and the thickness of the wall. As in flat membranes, thermal fluctuations are expected to influence the mechanical response of a deformed shell by renormalization of elastic constants. However, the fluctuations of thin shells are qualitatively different from those in flat elastic membranes due to the coupling of in-plane stretching modes and bending modes by the curvature. We study the deformations of these shells using Monte Carlo computer simulations and perturbation theory including the effects of curvature and external pressure. We show that thermal fluctuations reduce the critical buckling pressure and soften the mechanical response on point-like indentations.

Thermal fluctuations of microcapsules imply a softening of the mechanical response and a reducuction of the critical buckling pressure.

Last modified: 16/06/2012