Interfacial materials, such as foams, emulsions, are composed of many bubbles and drops, stabilized either by surfactants, polymers or colloids. The question we tackle is : « how do molecular interactions and interfacial dynamics in the surfactant layers control the macroscopic properties of foams, emulsions or capsules? ». We have a special interest in « reactive » systems, whose interfacial dynamics such as adsorption/desorption fluxes can be actively controled by an external parameter. Examples include photoswitchable surfactants or thermoresponsive colloids. From a fundamental point of view, these reactive systems are molecular tools that enable to study how a tiny modification at the molecular scale has consequences at the mesoscopic and macroscopic scales. Another topic of interest is the study of interfaces and wetting situations in which mass transfers and/or phase transitions occur, such as solidification by freezing, cross-linking, complexation, evaporation. These solidification processes lead to local modifications of the interfacial tension or the viscosity at the nanometric scale, which have a huge impact on macroscopic interfacial behaviours.
Assembling polymers at liquid interfaces for encapsulation
When polymer chains adsorb at liquid interfaces, they form « trains » and « loops », with a very fast adsorption/desorption dynamics between the adsorbed monomers in the « trains » and the non adsorbed monomers in the « loops ». When several polymer layers are assembled at a liquid interface, they form a membrane, which interfacial dynamics depends on the interactions at play, hence tailoring the interactions enables us to control the mechanical properites and stability of the membrane. Strong attractions between the layers, obtained by combining hydrogen bonds and hydrophobic interactions, hamper the adsorption/desorption dynamics and one obtains a viscoelastic membrane, which can be used to encapsulate droplets. Vanishing hydrogen interactions can be obtained by rising the pH which promotes the release and coalescence of the droplets.
Microfluidic probing of the interfacial rheology of LbL micro-capsules
We have developped a microfluidic chip that enables to probe the interfacial tension and interfacial moduli of capsules obtained by assembling polymer molecules layer-by-layer at the oil/water interface. The capsules go through a constriction followed by an elongation chamber in which an extensional flow is obtained. By varying the interactions between the polymer layers in the membrane as well as the anchoring energy of the first layer to the interface, we are able to tune the resistance of the membrane to shear and elongation or compression, which controls their behaviour in the constriction/elongation chambers. For weak interactions and anchoring energy, hence low interfacial shear and compression/elongation moduli, the capsules may elongate after the constriction and then return to a spherical shape with a dynamics which is driven by the interfacial tension and slowed down by viscous forces: following their relaxation enables to obtain the interfacial tension of the capsules. For strong interlayer interactions, the capsules become so rigid that they cannot be elongated after the constriction, they go directly from a flat shape (negative deformation) to a sphere : here the elastic modulus of the membrane drives the relaxation process. In the cases of strong anchoring energy and low interactions, the polymer layers may rearrange in the constriction, so that the capsules « forget » their spherical shape and remain flat out of the constriction. Once in the elongation chamber, the return to a spherical shape is driven by the interfacial tension while the elastic moduli hampers it.
Collaborations: M. Reyssat (ESPCI), T. Salez (LOMA), P. Perrin (ESPCI), N. Pantoustier (ESPCI), G. Fuller (Stanford), J. Vermant (ETH)
- « Microfluidic probing of complex interfacial rheology capsules », Tregouet, C., Salez, T., Monteux* C., Reyssat*, M., Soft Matter, 15 (13), 2782-2790, (2019) 10.1039/c8sm02507j
- Transient deformation of a droplet near a microfluidic constriction: A quantitative analysis, Trégouët, C., Salez, T., Monteux*, C. & Reyssat*, M. Phys. Rev. Fluids3, 053603 (2018). 10.1103/PhysRevFluids.3.053603
- Probing the adsorption/desorption of amphiphilic polymers at the air-water interface during large interfacial deformations Tregouet, C., Salez, T., Pantoustier, N., Perrin, P., Reyssat*, M., Monteux*, C., Soft Matter, (2019) 10.1039/c9sm00368a
- Trégouët, C. et al. Adsorption dynamics of hydrophobically modified polymers at an air-water interface. Eur. Phys. J. E 41, (2018).
- Dupré de Baubigny, J. et al. One-Step Fabrication of pH-Responsive Membranes and Microcapsules through Interfacial H-Bond Polymer Complexation. Sci. Rep. 7, (2017).
- Le Tirilly, S. et al. Interfacial Rheology of Hydrogen-Bonded Polymer Multilayers Assembled at Liquid Interfaces: Influence of Anchoring Energy and Hydrophobic Interactions. Langmuir 32, 6089–6096 (2016).
- Le Tirilly, S. et al. Interplay of Hydrogen Bonding and Hydrophobic Interactions to Control the Mechanical Properties of Polymer Multilayers at the Oil–Water Interface. ACS Macro Lett. 4, 25–29 (2015).
DYNAMICS OF COLLOIDS AT INTERFACES
Colloidal particles can adsorb at liquid interfaces and be used to stabilize foams or emulsions over long period of times. Unlike surfactants, colloidal particles adsorb irreversibly at interfaces and form highly rigid layers, which can protect bubbles from coalescence or coarsening. However the adsorption dynamics of colloidal particles is very slow, wich can be an issue in foaming or emulsifying processes, where large amounts of bubbles and drops have to be generated rapidly. Indeed colloidal particles diffuse slowly to interfaces and once they touch the interface, the progression of the particles toward their equilibrium position is very slow due to pinning/unpinning of the liquid interface on nanometric defects present at the surface of the particle. Below we give two examples of experimental systems for which the adsorption dynamics can be accelerated.
Adsorption dynamics at ultra low interfacial tension
We have studied the adsorption dynamics of micrometric particles at a water/water interface, obtained by demixion of two incompatible polymer solutions (dextran/gelatin). The interfacial tension is of the order of 10µN/m. We found that the adsorption dynamics is much faster than for standard oil/water interfaces with high interfacial tensions. This is due to the fact that the pinning energy of the contact line on the particles, which directly depends on the interfacial tension, is strongly reduced. However the adsorption of gelatin pancakes on the particles, entangled with other polymer chains in the semi-dilute network, may slow down the adsorption process.
Soft and responsive particles in thin liquid films
Soft PNiPAM microgel particles are micron sized particles which can spontaneously deform as they adsorb at liquid interfaces. The adsorption dynamics of such particles is therefore very fast. We study the link between the conformation of the particles at interfaces and the stability of the corresponding liquid interfaces and how this depends on the cross-linking density, hence softness of the particles. Studying the drainage of thin liquid films made with microgel solutions, we are able to deduce the conformation of the particles at the films interfaces. We also can learn whether the particles form a bridged monolayer or a stable bilayer. We find that the softest particles tend to spread at low concentrations while they form dense layers at high concentration in which the particles stretch perpendicularly to the interface, which stabilize the films against rupture.
P. Perrin, N. Sanson (ESPCI), V. Schmitt (CRPP), V. Ravaine (ENSCPB), J. Sprakel (Wageningen Univ), H. Tromp (Utrecht Univ), C. Collosqui (Stonybrook Univ.)
- « Colloidal particle adsorption at water/water interfaces with ultra-low interfacial tension », Keal, L., Colosqui*, CE., Tromp, H., Monteux, C., Phys. Rev. Lett.120, 208003 (2018) 10.1103/PhysRevLett.120.208003
- Influence of concentration and cross-linking density on the drainage of thin-liquid films containing microgels, L. Keal, V. Lapeyre, V. Schmitt, V. Ravaine, C. Monteux*, Soft Matter, 13(1), 170-180 (2017) – special issue emerging investigators
- Tracking the interfacial dynamics of PNiPAM soft microgels particles adsorbed at the air-water interface and in thin-liquid films, Y. Cohin, M. Fisson, K. Jourde, G. Fuller, N. Sanson, L. Talini and C. Monteux*, Rheologica Acta, 52 (5), p 445-454 (2013) special issue ‘special trends in Rheology’
- «Poly(N-isopropylacrylamide) Microgels at the Oil-Water Interface:Interfacial Properties as a Function of Temperature», C. Monteux*, C. Marlière, P. Paris, N. Pantoustier, N. Sanson, P. Perrin, Langmuir, 26, 13839-13846 (2010)
Photoresponsive surfactants and photofoams
Foams, are composed of many bubbles, stabilized by adsorbed surfactant molecules. The stability of interfacial materials over time or in response to a mechanical stress depends on the adsorption/desorption dynamics of the surfactant molecules, which depends on their shape or hydrophobicity, and plays a role at various scales, ie the drainage of a thin film, the coalescence of several bubbles… To understand and control the stability of interfacial materials, we used « photosensitive » surfactants which hydrophobicity can be tuned with UV or Blue illumination and we track the interfacial response from the microscopic to the macroscopic scale.
Azobenzen surfactants, which have a trans- conformation a rest, can isomerize into their cis trans when illuminated under blue or UV light, which induces a modification of their polarity: cis-isomers are more polar and surface active than the trans isomers. A modification of the surfactant’s polarity can have surprising consequences at the mesoscopic scale (thin films) and macroscopic scale (foams), from slower drainage in isolated films due to stabilizing Marangoni flows to foam rupture.
Collaboration : C. Tribet (ENS), F. Lequeux (ESPCI), I. Cantat (IP Rennes), A. Saint-Jalmes (IP Rennes)
- Chevallier, E. et al. Pumping-out photo-surfactants from an air–water interface using light. Soft Matter 7, 7866 (2011).
- Chevallier, E., Monteux, C., Lequeux, F. & Tribet, C. Photofoams: Remote Control of Foam Destabilization by Exposure to Light Using an Azobenzene Surfactant. Langmuir 28, 2308–2312 (2012).
- Chevallier, E., Saint-Jalmes, A., Cantat, I., Lequeux, F. & Monteux, C. Light induced flows opposing drainage in foams and thin-films using photosurfactants. Soft Matter 9, 7054 (2013).
- Mamane, A., Chevallier, E., Olanier, L., Lequeux, F. & Monteux, C. Optical control of surface forces and instabilities in foam films using photosurfactants. Soft Matter 13, 1299–1305 (2017).
Highly stable foams made with complex fluids
Increasing the stability of foams is a crucial issue in many applications such as surface decontamination or food products.We explore several possibilities to produce highly stable foams by increasing the viscosity of the bulk phase. For example highly stable foams can be obtained by stabilizing the foam with amphiphilic polymer chains which can be assembled into a non covalent network using hydrogen interactions. Cross-linking the polymer chains both at the interface and in bulk results in high interfacial and bulk viscosities which lead to highly stable foams.
Producing stable foams from highly viscous fluids is a challenge : during the foaming process, the high viscosity prevents incorporation of gas bubbles into the viscoelastic matrix. We investigate the role of rheological properties of the continuous phase on the foaming process with a special interest in shear thinning and yield stress fluids or dilute suspensions in which aggregates can break upon shear and reform at the interface.
- Deleurence, R., Saison, T., Lequeux, F. & Monteux, C. Time scales for drainage and imbibition in gellified foams: application to decontamination processes. Soft Matter, 11, 7032–7037 (2015).
- Deleurence, R., Saison, T., Lequeux, F. & Monteux, C. Foaming of Transient Polymer Hydrogels. ACS Omega 3, 1864–1870 (2018).
- Mixtures of latex and surfactants of opposite charge as interface stabilizers, R. Deleurence, C. Parneix, C. Monteux*, Soft Matter, 10, 7088-7095 (2014)
- Foamability and foam stability of silica/PEI gels, R. Deleurence, T. Saison, F. Lequeux, C. Monteux*, Colloids and Surfaces A, 534, 2-7 (2017)
FREEZING BUBBLES AND DROPS
The solidification of liquids containing bubbles and drops is of great interest in materials sciences. The fabrication of solid foams often starts with a precursor liquid foam which is then solidified. The fabrication of single crystals requires the exclusion of air bubbles during the crystallization process. We study a model system : the growth of ice crystals growing in a liquid containing bubbles or drops. During the growth of the ice, the surfactants, which are not soluble in the ice, accumulate at the ice/water interface. This concentration gradient induces forces that control the interaction between the drop and the ice crystals.
Collaboration : S. Deville (LSFC Cavaillon)
- A temperature-controlled stage for laser scanning confocal microscopy and case studies in materials science, Dedovets, D., Monteux, C. & Deville, S. Ultramicroscopy 195, 1–11 (2018). 10.1016/j.ultramic.2018.08.009
- Five-dimensional imaging of freezing emulsions with solute effects, Dedovets, D., Monteux, C. & Deville*, S. Science 360, 303–306 (2018). 10.1126/science.aar4503
The wetting dynamics of a drop on a substrate depends the flow and dynamics occuring at the scale of a few nanometers at the contact line between the solid and the drop. We study situations, inspired from industrial applications, where a mass transfer, caused by evaporation or substrate dissolution, occur very close to the contact line, which induces local variations of the viscosity or interfacial tensions, and modifies the wetting dynamics.
Wetting of soluble substrate
When a droplet spreads on a hydrophilic polymer film, the contact angle is not zero, as a dry polymer always present hydrocarbon moities at the air-solid interface. However, as the water drop spreads, water evaporating from the drop, diffuses into the film and hydrates it. The content of water in the film depends on the contact line velocity and thickness of the polymer film and controls the dynamic contact angle. In the case where the film is composed of a charged polymer, a gradient in osmotic pressure is created at the contact line, which can lead to the pinning of the contact line.
- Tay, A., Bendejacq, D., Monteux, C. & Lequeux, F. How does water wet a hydrosoluble substrate, Soft Matter 7, 6953 (2011).
- Tay, A., Monteux, C., Bendejacq, D. & Lequeux, F. How a coating is hydrated ahead of the advancing contact line of a volatile solvent droplet. Eur. Phys. J. E (2010) doi:10.1140/epje/i2010-10662-7.
- Monteux, C., Tay, A., Narita, T., De Wilde, Y. & Lequeux, F. The role of hydration in the wetting of a soluble polymer. Soft Matter 5, 3713 (2009).
- Tay, A., Lequeux, F., Bendejacq, D. & Monteux, C. Wetting properties of charged and uncharged polymeric coatings—effect of the osmotic pressure at the contact line. Soft Matter 7, 4715 (2011).
Wetting and evaporation
When a droplet containing colloidal particles is evaporating, the high rate of evaporation at the contact line and pinning of the contact line by the particles causes the accumulation of the solute at the contact line and to the formation of rings of colloids at the periphery of the drop. We have investigated the coupling between the pinning of the contact line and the evaporation flux in diverse situations. In the case of bidisperse colloidal dispersions, the small particles tend to segregate at the contact line. In the case of polymers solutions, the accumulation of polymer at the contact line induces a viscosity increase in the few nanometers close to the contact line, which controls the dynamic contact angle. As a consequence the dynamic contact angle depends on the relative humidity. We are now investigating the case of evaporating thin films of surfactant solutions, where additional Marangoni flows can either destabilize or stabilize the films, depending on the adsorption dynamics of the surfactants.
- Noirjean, C., Marcellini, M., Deville, S., Kodger, T. E. & Monteux, C. Dynamics and ordering of weakly Brownian particles in directional drying. Phys. Rev. Mater. 1, (2017).
- Liu, Y., Lee, D. Y., Monteux, C. & Crosby, A. J. Hyperbranched polymer structures via flexible blade flow coating. J. Polym. Sci. Part B Polym. Phys. 54, 32–37 (2016).
- Monteux, C. &Lequeux, F. Packing and Sorting Colloids at the Contact Line of a Drying Drop. Langmuir 27, 2917–2922 (2011)
- Monteux, C. & Lequeux, F. Packing and Sorting Colloids at the Contact Line of a Drying Drop. Langmuir 27, 2917–2922 (2011)
- Kajiya, T., Monteux, C., Narita, T., Lequeux, F. & Doi, M. Contact-Line Recession Leaving a Macroscopic Polymer Film in the Drying Droplets of Water−Poly( N , N -dimethylacrylamide) (PDMA) Solution. Langmuir25, 6934–6939 (2009)
- Monteux, C., Elmaallem, Y., Narita, T. & Lequeux, F. Advancing-drying droplets of polymer solutions: Local increase of the viscosity at the contact line. EPL Europhys. Lett.83, 34005 (2008)