activity controlled matter

Envoyé par kellay 
activity controlled matter
jeudi 9 février 2023 17:06:16
PhD Offer

Activity Controlled Matter

From a physics standpoint, active particles added to a soft material may generate internal stresses of sufficient intensity so as to inudce structural changes. These particles thus provide a unique opportunity to overcome the kinetic barriers at play in soft materials allowing to explore the energy landscape of such materials (gels, emulsions, foams, etc..) which are important in many applications. By providing means to actuate a soft material at the mesoscopic length-scales – at the origin of its complex rheology – active particles constitute a new and interesting tool to induce sharp or smooth changes and transitions in soft materials properties. Soft matter, including gels or surfactant phases is defined as matter whose structure is modified by thermal fluctuations. Additional activity due to the presence of active ingredients will necessarily influence the properties of soft matter systems. It has been shown both experimentally and theoretically that the coupling to an active bath modifies the structure of a soft material as well as its rheological properties.
The project concerns the interplay between activity and soft matter properties. In particular, the project will focus on characterizing the deformations of droplets and capsules filled by plankton cells or active Janus particles. Our basic hypothesis is that activity gives rise to an additional pressure and thus a higher effective Laplace pressure in droplets as seen in preliminary experiments in our group or a higher rigidity of the capsules. An emulsion made of such droplets or a gel made of such capsules filled with active ingredients will thus have its properties modified. By tuning the activity one can also hope to tune the properties of the materials.
In this project, the properties of the compartments will be characterized at equilibrium. The deformation of these compartments in a Taylor Mill configuration, for example, in the case of vesicles, provides means to determine the bending rigidity of the vesicles at a fixed temperature. For the case of droplets, one can measure the surface tension or Laplace pressure. In this project capsules, droplets and vesicles, of known interfacial properties as measured under equilibrium conditions will be used. These compartments will be filled with active particles or plankton cells and the link between the compartment properties and activity (velocity fluctuations of the active particles as measured separately) will be examined. In these experiments, the classical Taylor-mill configuration will be adapated to measure the effective mechanical pressure of well characterized active samples enclosed in droplets or capsules of know properties. Active particles (artificial or cells) will be encapsulated in deformable compartments of different natures using classical means or microfluidics. The spatial organization of the particles and their collective dynamics will have to be examined. Based on preliminary experiments using light-actuated colloids, we expect to obtain measurements of an active analog of the Pressure. In the long run, the plankton or active particle filled capsules are potentially mobile (by analogy with previous experiments in our group) and can thus exert larger stresses than the individual cells at larger scales. Such mobile artificial cell like structures will be potentially useful in acting on soft matter properties. Additional experiments on characterizing the properties of the active baths used (collections of plankton cells or collections of artificial active particles) will be carried out. This will consist in measurements of the velocity at the inidividual particle level but also at the collective level. This work is part of a larger ANR project (ACM) in collaboration with three labs at the College de France, ESPCI and CRPP.

Note : Ce travail se fera dans le contexte de l'ANR collaborative ACM en collaboration avec A. Colin de l'ESPCI, J. C. Baret du CRPP, et de J. F. Joanny du Collège de France et de l'Institut Curie. Le travail de thèse se fera sous la direction de H. Kellay et de J. F. Boudet dans le groupe Instabilités et Turbulence de l'équipe Matière Molle et Biophysique du Laboratoire Ondes et Matière d'Aquitaine (UMR 5798 CNRS/U. Bordeaux) situé sur le campus de Talence de l'université de Bordeaux.

Related Publications from the group :
J. F. Boudet et al, Phys. Rev. Res. 2022
K. Xie et al, Phys. Rev. Lett. 2022
J. F. Boudet et al. Science Robotics, 2021
T. Barois et al. Phys. Rev. Lett. 2020
A. Deblais et al. Phys. Rev. Lett. 2018

Contact :

Interested candidates should send their CV, a letter of motivation, and contact information of two or more references.