■ Topological living matter

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Description of the PhD project

In cells, thousands of nanometre-sized molecular motors coordinate themselves to carry out mechanical tasks at much larger length scales, such as cell motility, division and replication. They do so by intertwining similar protein components that operate either as scaffold constituents or force-generators. The composite assembled in this way is a delicate, natural active material that exhibits sought-after properties such as autonomous motility, internally generated flows and self-organized beating. A replicated in-vitro active system has recently been developed by mixing bundled polymerized tubulin filaments (microtubules) with kinesin motors. When this biomimetic material is confined to a surface, the microtubules develop long range orientational order. That, coupled with the large-scale self-organized flows in the system, results in the formation of a two-dimensional active nematic (AN) material, whose fascinating properties have just started to be experimentally explored.

Recent results from the SOC&SAM team at the University of Barcelona have shown that the turbulent active flows typically observed in ANs can be controlled by confining the system to a water / passive-liquid-crystal (PLC) interface. Remarkably, the structural patterns on the PLC are imprinted on the active flow, producing regular dynamic structures. At ESPCI, we have recently developed a rich platform to produce tunable high-order hierarchical structures in PLCs by imposing topological constrains to the system, specifically by confining the PLS to a thin spherical shell. The goal of this PhD is to extend these studies to ANs.

Confinement and curvature typically induce the formation of topological defects in the nematic material, which are singular points in the orientational field. Unlike in equilibrium systems, where defects are largely static structures, in ANs defects move spontaneously and can be described as self-propelled particles. For instance, ordinary spherical AN droplets exhibit a periodic state where defects oscillate between two different configurations. The physics emerging when coating a AN droplet with a PLC spherical shell is "terra incognita" that we would like to explore. To produce these systems, we will use double emulsions where a AN droplet is encapsulated inside a larger PLC droplet through microfluidic techniques. The richness of defect structures observed in PLC shells let us envisage a myriad of new dynamical structures and collective behaviors emerging from the coupling between active and passive defects. On the other hand, the defect-mediated self-organization of dispersed droplets or solid particles in PLCs has recently had a large impact. However, this idea has never been formulated before in relation to the LC-mediated self-organization of active inclusions. We will study these active inclusions and the possibility to synchronize droplet-based cortical flows using a passive or actuated LC as the coupling matrix.


Active matter, Liquid crystals, Topological defects, Bio-inspired materials, Confined systems, Droplets, Nematic colloids, Self-assembly

Research unit


Description of the research Unit/subunit

The research project will be mainly carried out at "Gulliver" (https://www.gulliver.espci.fr), which is a mixed research unit ESPCI Paris and CNRS (UMR 7083, Director E. Raphaël). Gulliver is an internationally renowned soft matter laboratory, composed by 15 permanent researchers with expertise in physics, physico-chemistry and molecular biology. Over the last 5 years, the lab has hosted 37 Postdocs, 34 PhD students, 110 Master students and 25 visiting Professors. Gulliver is divided in four subunits : "Theoretical Physical Chemistry", "Microfluidics MEMs and Nanostructures", "Collective Effects and Soft Matter" (EC2M), and "Molecular Systems and Programs". The PhD student will part of EC2M. The research activities of the team focus on collective effects in active systems, topological defects in LCs, particle self-assembly, and microfluidics for biotechnology. The lab is equipped with state-of-the-art experimental techniques such as ultra-high speed confocal microscopy, submicron resolution 3D printing, or a top-class microfluidic platform. This intellectual environment is ideal for promoting both excellent research and first-class student training in Soft Matter.

Name of the supervisor
Teresa Lopez-Leon

Name of the co supervisor
Jordi Ignés-Mullol

3i Aspects of the proposal

The goal of this project is to combine bio-inspired active materials with synthetic LCs and topological constrains to produce new topological active materials. This project is inherently interdisciplinary, since it requires combining Biology and Chemistry, to produce the active material, with Physics and Mathematics, to understand its macroscopic behavior. One of the current goals in nanotechnology (one of the six KETs identified by the EU as “priority for European industrial policy”) is to be able to create, control, and assembly photonic crystals and metamaterials — materials that owe their properties not to the microscopic constituents but, rather, to the couplings, connections, and geometry. Topological defects offer promising perspectives in this sense, since they are powerful tools to make nanoparticles self-assemble into complex 3D architectures. Beautiful realizations of such architectures have been obtained by inducing defect-mediated interactions between solid particles or droplets in a PLC. One of our goals is to substitute the PLC by an active LC, and study the properties of the resulting material. The active nature of the emerging defects could lead to adaptability, flexibility, and even complex collective behaviors that could result into the emergence of smart biocompatible materials. Here, we will be stepping into unknown territories, opening opportunities for innovation and intersectoriality. The company “Advanced Nanotechnologies” (http://www.advancednanotechnologies.com) has shown direct interest in the outcome of this research project. The project is set in the framework of an international collaboration between ESPCI Paris (France) and the University of Barcelona (Spain). The PhD student will be supervised by Dr. Lopez-Leon from ESPCI Paris and co-supervised by Prof. Ignés-Mullol from UB, with the possibility of co-accreditation of PhD degrees between the two institutions. We anticipate the task of the PhD candidate to involve yearly visits to Barcelona (about 4 months per year total, in one or two visits). The student will also benefit from several international consortia in which the Barcelona or Paris teams are involved, such as the H2020-FETOPEN project “ABIOMATER”, which brings together a team of researchers across Europe to develop a new class of metamaterials, or the H2020-MSCA ITN network “NANOTRANS”, which aims at understanding the transport of fluids and colloids at the nanoscale.

Expected Profile of the candidate

Applicants must be in possession or finalizing their Master’s degree or equivalent/postgraduate degree. Experimental experience on Soft Matter, and more specifically on liquid crystals, active matter, or microfluidics will be appreciated. The candidate should be comfortable in multinational, multilingual working environments and willing to stay abroad for long periods of time (two 2-month visits to Barcelona per year). We will use our international networks for diffusing the potential PhD opportunity to top-class European candidates. In particular, Prof. Ignés-Mullol participates in the master of Applied Materials Chemistry and in the master of Nanoscience and Nanotechnology at the UB, both incorporating top-level local and international students that could be excellent potential candidates.

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