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Fabricating colloidal materials with microfluidics

Periodic Reporting for period 3 - Microflusa (Fabricating colloidal materials with microfluidics)

Reporting period: 2018-03-01 to 2020-02-29

Today, in the field of colloidal science, much effort is dedicated to the synthesis of complex building blocks mimicking molecular structures. By assembling them in large quantities, our hope is to elaborate innovative materials possessing interesting photonic, microwave or acoustic properties. Among these materials, photonic structures with a complete forbidden band gap in the visible range deserve particular attention, because they have the ability to drive light in channels, in a way analogous to microelectronics with electrons. If such materials could be realized under low cost conditions, one could expect a breakthrough in the field of optoelectronics. Today, materials such as these, do not exist. They could be made, in principle, by using standard techniques of microfabrication, such as deposition, etching, high resolution photolithography, or 3D printing, assuming that in the future, resolutions will become appropriate. However, these techniques are slow and costly, and thereby are not compatible with obtaining low cost devices. This is where colloidal approach comes into play. The colloidal approach consists in elaborating elementary blocks, and using a self assembly process to make materials. In essence, this consists in dropping thousands of complex particles in a vessel and wait for spontaneous organization to take place. The MicroFlusa project stands in this framework.

ESPCI/MMN discovered, prior to the beginning, a novel method for making colloidal building blocks using microfluidic droplets.

To move from these building blocks to the elaboration of materials, a number of tasks must be performed. These tasks are cross-disciplinary, because they concern issues related to the fields of chemistry, physico-chemistry, hydrodynamics, optics, at experimental, theoretical and numerical levels. This is why MICROFLUSA gathered a consortium including chemists, hydrodynamicists (theory, numerics and experiment) and optics experts.
The first year is dedicated in establishing the foundations: series of experiments were carried out in order to analyze the plug formation process in the particular geometry we use in the project. This geometry, in which channel heights are much smaller than channel widths, allows to minimize volumes and thus cluster sizes. Thereby, with this approach, it becomes easier reach micrometer dimensions, closer to the optical range we target. In such a geometry, we discovered a new, unexpected regime, that is worth being analyzed from a fundamental prospective. This regime produces plugs of sizes diminishing with the speed, according to a power law that we did not succeed to explain. These observations challenge our Technion colleagues, and we hope, in the future to offer a full description of this regime, including theory and experiments.

This work has been reinforced by numerical simulations made in Stockholm. The KTH team carried out hydrodynamic direct simulations that could reproduce well the formation of well defined, symmetric, building blocks from featureless clusters of droplets. These results are encouraging. However, the agreement between numerics and experiments was obtained at the expense of assimilating droplets to rigid spheres and using a model of Van der Waals forces incorporating unrealistically large spatial extensions. In practice, micrometers instead of nanometers. Based on this model, numerics shows how droplet clusters, mutually attracted, evolve towards symmetric shapes, trimers, quadrimers, ... exactly as in the experiment. Further work is needed to figure out the reason why so large Van der Waals forces must be used to mimick the experiments.

In parallel with these hydrodynamic contributions, Technion has proposed an interesting structure that could give rise to materials with forbidden band gaps, easier to fabricate than in previous work. The material is in form of a square lattice of droplets encapsulating dimers, aligned in a prescribed direction. These structures develop a complete band gap. The advantage of this approach is twofold: the properties are obtained with the direct structure, and, thanks to the droplet-based template, which can be produced with high crystallinity, the process is suitable for minimizing the number structural defects. One limitation of this structure is the smallness of the width of the band gap, which, in the best case, does not exceed 8%, a figure significantly below the diamond structure. This may raise issues concerning the sensitivity of the material behavior with respect to mechanical distorsions, defects, however small their number can be.

In the second year, we engaged discussions with theoricians interested in hyperuniformity, a very recent concept in the field of colloidal science. These discussions emphasized on the fact that disordered materials of a certain class (hyperuniform materials) can open complete photonic band gaps, even in the presence of disorder. This remark facilitates the fabrication of such materials, by allowing the presence of disorder in their creation process, without loss of their photonic properties. We investigated this new pathway, and showed, by using microfluidics again, that such a material, produced by assembling droplets of different sizes, can be optimized from the viewpoint of hyperuniformity. We also showed that, after drying, these materials – thus looking as foams - have structures favorable for opening band gaps.

In the meantime progress was made on chemistry (by synthesizing new surfactants), theory (explaining droplet formation in shallow cells), numerics (investigating the origin of dipolar interaction) along with optics (by investigating the photonic properties of a variety of structures, hyperuniform or not.
During the first two years, six results going beyond the state-of-the art have been obtained.

1) We discovered a new, unexpected regime, of production of droplets in T-junctions, associated to a power law that remains to be explained. Owing to the importance of this geometry in the domain of droplet based microfluidics, this result should generate interest in the community.

2) We explained qualitatively the reshapening of droplet aggregates in a hydrodynamic channel.

3) We discovered a new structure that develops a complete band gap in 3D. The advantage of this approach is twofold: the properties are obtained with the direct structure, and, owing to its simplicity, the self-assembly process could be deployed without generating a significant number of defects. This structure may be the leading edge of a new family of structures giving rise to interesting photonic properties. Further analysis however showed the width of the band gap is comparable to a fcc structure (8%), which is too small to hope for industrial applications.

4) We fabricated hyperuniform bidisperse emulsions, with different characteristics, screened their properties and optimized their hyperuniform performances.

5) We performed an exhaustive study of the behavior of droplet assemblies, that reproduce well the experimental observations, at a qualitative level.

6) Progress in the theory of droplet formation in Helle Shaw cell has been made

At the moment, we made scientific contributions beyond the state-of-the art. Societal impact is not clearly visible, but it stands in the perspective of this work. Scientific impacts concern the fields of fluid mechanics and photonic materials.
WP5 Simulation: Clustering Droplets