Periodic Reporting for period 2 - CoPEC (Colloidal particles in elasto-capillary fields)
Okres sprawozdawczy: 2021-01-01 do 2021-12-31
The “CoPEC” project was a fundamental research project designed to advance knowledge in the field of soft matter Physics, and more particularly in the area of colloids and complex fluids interfaces. The main task of the proposal was to investigate the physical properties of colloidal particles evolving in elastocapillary fields, which can be realized by considering, e.g. particles attached to liquid crystal (LC) fluid interfaces. Such systems remain largely unexplored today and represent a new type of material whose properties are anticipated to be mainly governed by the coupling between capillary and elastic phenomena. The former originates from the surface tension inherent to any fluid interface, whereas the latter is intrinsic to all liquid crystal phases. These phenomena have always been considered separately in previous works, and consequently, examining their intimate coupling has been one of the salient and innovative aspects of the “CoPEC” project. Indeed, elasto-capillary couplings may lead to novel colloidal interactions and the subsequent discovery of new collective properties which could be exploited to design materials with yet unknown important functions. Potential areas of impact include optical applications such as the next generation of ‘smart’ PDLC (Polymer Dispersed Liquid Crystals) windows or drug delivery systems at the micro- or sub-micrometer scale for the pharmaceutical industry.
The project was carried out using numerical simulations based on continuum theories. The overall objective was to develop computer models to gain basic knowledge on the behavior of colloidal particles adsorbed at liquid crystal interfaces. Understanding collective phenomena, which are essential to make predictions on potentially interesting applications, was one of the final scientific targets of the project. But prior to addressing multi-particle systems, a large part of the project first focused on gaining general knowledge on (i) the behavior of a single particle, and (ii) pair interaction potentials.
Overall, “CoPEC” fulfilled its main objective, i.e. new computer codes could be designed and developed to improve our understanding of particles straddling LC interfaces. Unexpected results (see below), essentially at the single particle level, were unveiled due to the aforementioned elastocapillary coupling, allowing new insights to be gained. However, a great deal of efforts are still needed to fully characterize such systems, whose physical properties are very rich due to the numerous intertwined effects (e.g. capillarity, elasticity, multiphase flows).
The project was carried out using numerical simulations based on continuum theories. The overall objective was to develop computer models to gain basic knowledge on the behavior of colloidal particles adsorbed at liquid crystal interfaces. Understanding collective phenomena, which are essential to make predictions on potentially interesting applications, was one of the final scientific targets of the project. But prior to addressing multi-particle systems, a large part of the project first focused on gaining general knowledge on (i) the behavior of a single particle, and (ii) pair interaction potentials.
Overall, “CoPEC” fulfilled its main objective, i.e. new computer codes could be designed and developed to improve our understanding of particles straddling LC interfaces. Unexpected results (see below), essentially at the single particle level, were unveiled due to the aforementioned elastocapillary coupling, allowing new insights to be gained. However, a great deal of efforts are still needed to fully characterize such systems, whose physical properties are very rich due to the numerous intertwined effects (e.g. capillarity, elasticity, multiphase flows).
The original work plan was composed of four Work Packages (WP). The first one dealt with the global management of the project and also included an intense learning and training phase. The remaining three WPs were more technical and of increasing complexity. In agreement with the work plan, I spent the first 6 months or so gaining new scientific knowledge, mostly on Phase-Field (PF) methods, and getting trained in numerical simulations based on finite elements (WP1). Afterwards, I began addressing the tasks of WP2, which focuses on the behavior of a single particle trapped at a fluid interface. The case of an isotropic/isotropic (referred to as Iso-Iso hereafter) interface (no LC here) was tackled first in order to validate our approach. We achieved very good agreement with literature data for both static (absence of prescribed external flows) and dynamic (computation of drag forces) situations. Switching to an Iso-LC interface made things a lot more challenging for we had to deal with one of the core issues of the project, i.e. the so-called elastocapillary coupling. We considered the simplest of all LC phases, i.e. the nematic (N) phase. Building on existing theoretical models, we coupled the PF method, which describes the dynamics of a fluid interface (surface tension), to a tensor-based theory able to account for the partial ordering of the N phase (elastic properties) along with the possible formation of topological defects. We faced numerous issues and it took us a lot more time than we originally thought to troubleshoot and validate our methodology on benchmark examples. With our validated model, we were able to explore both the static (floating particle configurations) and dynamic (computation of drag forces) behaviors of a particle trapped at an Iso-N interface. Our results show that the interplay of elastic distortions, surface tension and boundary conditions (anchoring of LC), together with the two-way coupling between the flow and the nematic fields, govern the response of such systems. WP2 can be considered finished despite a few ongoing, but soon ending, side-studies.
Extending the previous studies to the case of curved Iso-Iso and Iso-N interfaces was the matter of WP3. Unfortunately, only embryonic explorations could be performed at such interfaces, mainly because of a lack of time.
Finally, the first task of the last work package (WP4) could be partially addressed. This WP focuses on collective phenomena involving two or more particles straddling an Iso-N interface. As in WP2, we first successfully benchmarked our model for an Iso-Iso interface with two particles undergoing an attractive capillary interaction. We then extended our calculations to an Iso-N interface and obtained preliminary results highlighting the possibility of having attractive capillary interactions competing with repulsive elastic interactions. However, this promising investigation could not be pursued further due to a lack of time.
The results collected so far allowed us to publish several papers, as described in the next section.
Extending the previous studies to the case of curved Iso-Iso and Iso-N interfaces was the matter of WP3. Unfortunately, only embryonic explorations could be performed at such interfaces, mainly because of a lack of time.
Finally, the first task of the last work package (WP4) could be partially addressed. This WP focuses on collective phenomena involving two or more particles straddling an Iso-N interface. As in WP2, we first successfully benchmarked our model for an Iso-Iso interface with two particles undergoing an attractive capillary interaction. We then extended our calculations to an Iso-N interface and obtained preliminary results highlighting the possibility of having attractive capillary interactions competing with repulsive elastic interactions. However, this promising investigation could not be pursued further due to a lack of time.
The results collected so far allowed us to publish several papers, as described in the next section.
Following on the aforementioned findings, “CoPEC” enabled to advance knowledge in the field since three papers have already been published, and another one has been submitted for publication. Briefly, the first paper was published in the European Physical Journal E (vol. 43, p13, (2020), DOI 10.1140/epje/i2020-11936-1) and pertains to the influence of interfacial deformations on the drag force exerted on a particle straddling an Iso-Iso fluid interface (WP2). The second paper appeared in Physical Review E (vol. 103, 022706; (2021), DOI: 10.1103/PhysRevE.103.022706) and concerns the implementation of the elasto-capillary coupling (WP2), making it possible to explore rich and novel phenomena. The third paper (European Physical Journal E, vol. 44, p30, (2021), DOI 10.1140/epje/s10189-021-00025-w) focuses on the validation work carried out in WP4 for Iso-Iso interfaces. We unexpectedly came across a novel phenomenon related to pair capillary interactions occurring between translating and rotating particles. Finally, an additional paper connected to the work performed in WP2 has been submitted for publication in Physical Review E. This important contribution contains all the results garnered for more than a year and a half on the static and dynamic studies of a single particle trapped at the Iso-N interface.
As a fundamental research project, “CoPEC” is not expected to yield significant socio-economic impacts nor have particular societal implications in the foreseeable future. Although “CoPEC” might yield promising applications in a variety of areas, as pointed out in the proposal, the relevance and importance of such concrete impacts are difficult to assess.
As a fundamental research project, “CoPEC” is not expected to yield significant socio-economic impacts nor have particular societal implications in the foreseeable future. Although “CoPEC” might yield promising applications in a variety of areas, as pointed out in the proposal, the relevance and importance of such concrete impacts are difficult to assess.