Surfoids : Colloids @ Fluid Interfaces

The interface between two fluids is extremely efficient to trap particles ranging from nanometer to millimeter sizes. In the past this strong 2D confinement has been used to address fundamental problems of condensed matter physics considering colloids as “big” atoms. Nowadays the interest for these systems is enriched by their large impact for applications and industrial processing. In particular making functional materials by colloidal self-assembly at the interface needs the control and tuning of forces on the particles (a). In addition the optimization of environmental processes as flotation in waste water treatments requires the extension of all these studies to more complex colloidal morphologies beyond the spherical one (b). Finally, the use of particles as stabilizers in cosmetic and food foams demands to better understand the interfacial dynamical properties of these complex interfaces (c). The Soft Matter team is committed to all these new highways of research in this field.

(a) Control & tuning of forces on colloids trapped at fluid interfaces

Curvature controlled forces

When the fluid interface is non-uniformly curved due to boundary conditions on the walls containing the fluids a coupling between the colloids and the local curvatures is expected. Models predict that a force proportional to the gradient of the local Gaussian curvature of the interface acts on the trapped particle. In order to test these theoretical predictions we design by photo-lithography sub-millimetric cuvettes to create air /liquid interfaces with an imposed gradient of Guassian curvature. Using a null force method where the know gravity force competes with the curvature force we are able to measure extremely tiny curvature forces down to few femto-Newton. Our experimental results are in good agreement with theoretical predictions.

• Ch. Blanc, D. Fedorenko, M. Gross, M.In, M. Abkarian, M. Gharbi, J.-B. Fournier, P.Galatola, M.Nobili, Capillary force on a sphere trapped at a fluid interface of arbitrary shape, submitted (2012).

Elasticity controlled forces

When one of the fluids is structured as in the case of a liquid crystal, elastic and defect mediated forces between the colloids appear. By optical tweezer and particle tracking we are able to measure the pair-wise interaction between silica particles trapped at a flat air/liquid crystal interface. The long-range asymptotic behaviour of the interaction is well fitted by a model we develop which takes just into account the nematic elasticity. This interaction is at the origin of the measured hexagonal patterns (see Figure below). At shorter distance the interaction is more complex being governed by defect transformation and recombination. These effects have drastic consequences : for example they transform the crystal pattern in a glassy structure.

• Behavior of colloidal particles at a nematic liquid crystal interface, M. A. Gharbi, M. Nobili, M. In, G. Prévot, P. Galatola, J.-B.Fournier and Ch. Blanc, Soft Matter, 7, 1467 (2011).
• Interactions of bulk dislinations with colloids trapped at a nematic liquid crystal interface, M. A. Gharbi, M. Nobili, Ch. Blanc, to be submitted (2012).

Elasticity and defect mediated forces can also be employed to control the position of spherical colloids on curved water/liquid crystal interfaces. With this aim we have made by microfluidics liquid crystal shells in water/LC/water double emulsions. The geometrical frustration of the nematic order imposes the presence of topological defects. We studied the equilibrium position of such defect and how to control it with temperature close to a nematic-smectic transition (see figure below). Topological constraints and liquid crystal elasticity yield fascinating structures which control the position of solid particles on the sphere. We currently explore how these phenomena could be used to create micrometric spheres with controlled valences and directionalities.

• T. Lopez-Leon, A. Fernando-Nieves, M. Nobili, C. Blanc, Nematic-smectic transition in spherical shells , Phys. Rev. Lett., 106, 247802 (2011) .
• T. Lopez-Leon, A. Fernando-Nieves, M. Nobili, C. Blanc, Smectic Shells, Journal of Physics : Condensed Matter, 24 , 284122 (2012).
• D Sec,T. Lopez-Leon,M. Nobili, Ch. Blanc, A. Fernandez-Nieves,M. Ravnik,S. Zumer, "Defect trajectories in nematic shells : Role of elastic anisotropy and thickness heterogeneity" , Phys. Rev. E, 86, 020705(R) (2012). M.A. Gharbi, D. Sec, T. Lopez-Leon, M. Nobili, S. Zumer, and Ch. Blanc, submitted (2012)

(b)Toward more “natural” colloidal morphologies

Synthetically designed micrometer-sized spheres can be trapped at the interface between two fluids without deforming it. The Young’law, which imposes the contact angle at the triple line, simply imposes the vertical position of the bead (see Fig.1-left). This is no longer the case with other more natural shapes. In a first attempt to approach them, we synthetized ellipsoidal colloids (Fig. 1-right) ; in this case the contact angle condition at the triple line yields a deformation of the fluid-fluid interface. Using interferometry and tracking techniques we are able to measure the ellipsoid dynamics and the in-situ interface deformation. We found that the interface and triple line deformations have important consequences on the particle dynamics dramatically enhancing its viscous drag.

Fig. 1 : (Left) Spherical particles can be trapped at fluid interfaces without deformation. The contact angle θ imposes the vertical position h of the particle. (Right) Phase shift interferometry reveals that micron-sized ellipsoidal particles deform the interface at the nanometer scale. This gives rise to long distance quadrupolar capillary interactions between particles.

(c) 2D visco-elasticity and anisotropy

In order to measure the dynamical properties of a fluid interface, trapped colloids can be used as micrometric probes in the well-known technique of micro-rheology. We recently start to use this technique to measure the viscous properties of Gemini-like surfactants adsorbed at an air/water surface. The used surfactant is soluble in water and micellizes into worms-like micelles. We found that the surface behavior is largely governed by the bulk viscosity. We plan to extend these studies to insoluble Gemini surfactants, in order to explore the possible visco-elasticity and anisotropy of these compounds.