Interaction and structuration of membrane-binding and membrane-excluding colloidal particles in lamellar phases

Our paper has just been published in Soft Matter!

Membrane-Mediated Interactions

Our review chapter (with Anne-Florence Bitbol and Jean-Baptiste Fournier) has just appeared in Physics of Biological Membranes (Patricia Bassereau and Pierre Sens, eds.)

Internship proposals

I have two internship openings at the M2 level for 2017. Contact me for more information.

Optical and structural properties of polymer - nanoparticles composites
Supervisors: Emmanuel Beaudoin and Doru Constantin

Viral nanocages assembled around gold nanoparticles
Supervisors: Doru Constantin and Guillaume Tresset

What is an order parameter?

For those of us working with liquid crystals, the answer tends to be fairly automatic: "the average of the second Legendre polynomial over the orientation distribution". Only in a second step do we think to qualify the definition: it concerns the quadrupolar order parameter (call it S) in a three-dimensional system. S = 0 for isotropic orientation, S = 1 when the molecules are perfectly oriented along an axis (the director) and S = –1/2 when they are all perpendicular to the director.

Whether it is the appropriate one depends on the problem at hand: if the particles constituting the system do not have inversion symmetry we should probably use the dipolar order parameter. It is less obvious that, although S describes well the tendency of molecules to align along an axis or perpendicular to it, it is not appropriate for any preferred angle: if all molecules make the "magic angle" θ_m = arccos(1/√3) ≅ 54.7° with the director, S is again zero so it cannot help distinguish between this situation and a purely isotropic distribution. One should then resort to the octupolar order parameter (or possibly a combination of quadrupolar and octupolar terms, for other angles?)

It may be useful to look at order parameters pragmatically, as Landau did for his theory of phase transitions: they are constructed such as to be zero in one phase and finite in another one. It is up to us to identify the phases and to define the most convenient parameter with regard for the particular system but also for the information we want to extract.

Defining nematic viscosities: Mięsowicz and Leslie-Ericksen

Isotropic fluids only have two viscosities, intervening in shear and extensional deformations. For anisotropic media, such as nematic liquid crystals, more coefficients are needed, as shown by Mięsowicz in the late '30s: three shear viscosities, labeled \(\eta_1\) to \(\eta_3\), a fourth one \(\eta_{12}\) introduced later by Helfrich and a rotational viscosity, \(\gamma_1\). We do not worry here about extensional deformations.

The whole topic was put on a solid theoretical basis in the '60s by Leslie and Ericksen [brief and clear presentation here] who introduced six coefficients (\(\alpha_1\) to \(\alpha_6\)), only five of which are independent. As one can expect from dimensional analysis, the two sets of viscosities, \(\left \lbrace \eta_i, \gamma _1 \right \rbrace\) and \(\left \lbrace \alpha_j \right \rbrace\) are linearly related.

I only recently realized, while discussing with my former PhD advisor, that the difference between the two definitions is deeper than an arbitrary linear transformation. Mięsowicz had in mind clear experimental configurations, defined by the relative orientation of director, velocity and velocity gradient, while Leslie and Ericksen adopt a more formal approach, based on generalized hydrodynamics, as in the paper of Martin, Parodi and Pershan.

The twist (so to speak) is that the theoretical approach gives a clearer view of the various modes and the constraints on the coefficients, while the Mięsowicz configurations are very difficult to achieve in practice, precisely due to the coupling between flow and director orientation.

Postdoc position: numerical simulations for biophysics

A post-doctoral fellowship is available at the MSC laboratory of the Paris Diderot University (Paris, France), in the framework of our ANR project. Apply before October 1st. The contract should begin on January 1st 2016 at the latest.

Title: Modelling the many-body interactions between protein inclusions in cell membranes

Gross salary: ~ 2500 €/month (varies with seniority).

Duration: ~ 12 months.

Summary of the research topic: The MSC laboratory is a research unit at the very heart of Paris working on three main axes: non-linear physics, soft-matter and interface between physics, biology and medicine. The subject proposed here is part of a wider research effort, pursued in collaboration with three other laboratories from the Paris area, in the framework of a project financed by the ANR (French grant agency). In particular, the successful candidate will work in close collaboration with an experimental group in the Laboratory of Solid State Physics (LPS, Orsay).

"Universal" Casimir forces

What is remarkable about the quantum Casimir force between perfect conductors is its very simple formula, which depends only on the geometry and on universal constants. Would this force be the same in the presence of different interactions?

Silver and gold nanoparticles in smectic mesophases

Our paper on the structure and optical properties of small noble metal nanoparticles was published in Chemical Communications!

 Wiktor Lewandowski et al., Chem. Commun. 2013

PhD position in soft matter/biophysics

I am looking for a PhD student (the financing should start in fall 2013). The topic is the experimental study of the interaction between membrane inclusions (part of the ANR project MEMINT). See the offer.

The candidate should have a background in soft matter physics, biophysics or physical chemistry. The project is essentially experimental, involving system formulation, sample preparation and alignment, preliminary characterization and detailed study by small-angle X-ray scattering, using both laboratory and synchrotron sources. Experience with data treatment techniques is not required, but can be an asset.

Your work will fit into a multi-disciplinary collaboration, involving specialists in soft matter physics and biophysics (both experimentalists and theorists) as well as chemists. You should be a quick learner and communicate well.

The morphology of C12E8 micelles

Our work on the structural analysis of the isotropic phase of the C₁₂E₈/H₂O mixture was (finally) published.

 

Structure, thermodynamics and dynamics of the isotropic phase of spherical non-ionic surfactant micelles

Kévin Tse-Ve-Koon, Nicolas Tremblay, Doru Constantin, and Éric Freyssingeas

Journal of Colloid and Interface Science, 393, 161–173 (2013).