The Microflusa project receives funding from the European Union’s Horizon 2020 research and innovation programme under Grant Agreement No. 664823
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1 year MICROFLUSA: Press Release

Aug 31, 2016

One-year of new and revolutionary colloidal materials

Giving shape to new materials...


1 September 2016:  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, we hope to elaborate innovative materials possessing interesting photonic, microwave or acoustic properties. ESPCI has discovered before the beginning of the project, a novel method for making colloidal building blocks. The method consists of emitting droplets aggregates in a microfluidic channel and, by harnessing hydrodynamic interactions, reshape these clusters into well controlled structures, such as trimers, tetrahedrons. These structures are remarkably monodisperse, and can be produced without defect in quantities of thousands per hour.


To move from these building blocks to the elaboration of materials, one must perform a number of tasks. 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 multidisciplinary consortium including chemists, hydrodynamicists (theory, numerics and experiment) and optics experts.


The first year of the Microflusa project was dedicated to establishing the foundations: series of experiments that will be 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.


This work has been reinforced by numerical simulations performed by the KTH team in Stockholm. KTH 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 extremely 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 show 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 mimic 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 works. 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 distortions, defects, however small their number can be.


During the first year, three 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.

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