Microfluidic study of the dynamics of crystal nucleation and partial coalescence in mono-disperse two-dimensional oil-in-water emulsions

Date

2019-12

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Abstract

Crystal nucleation and partial coalescence in oil-in-water emulsions play a critical role in engineering emulsion-based functional materials. Revealing the role of inter-droplet interactions on the mechanisms of both these processes is the key to the development of predictive tools for the optimal emulsion control and development. To this goal, we developed a temperature-controlled microfluidic platform to generate size-tunable emulsions and visualize the nucleation, and partial coalescence processes in 2-dimensional (2D) ordered and disordered multi-droplet arrays at the droplet level for the first time. Applying our novel microfluidic approach, during the cooling step of the thermal cycling, we aimed at finding whether nucleation in one drop occurs independently of the nucleation of its neighbors, is promoted by nucleation events in neighbors or is inhibited by them. So, we created 2D arrays of hexagonally packed droplets consisting of regular hexagonal cluster units of liquid droplets with one central drop and six immediate neighbors. As the droplets crystallized, these liquid hexagonal droplet clusters transformed to hexagonal clusters of solid drops. Then at each solid fraction, we calculated the ratio of the number of these solid hexagonal clusters to the maximum number of them, which occurs in a fully crystallized array. Then as a benchmark, we simulated the nucleation process in a 2D hexagonal droplet array, in which crystallized droplets at each solid fraction are located randomly. At any solid fraction, the normalized number of solid hexagonal clusters in the experiments exceeded the average value of 100 random simulations, indicating that solid droplets promote nucleation in their neighbors. We controlled the impact of inter-droplet interactions by varying the configuration of the emulsion to understand the origin of the promotion of nucleation. Also, applying Voronoi tessellation to monitor the distribution of solid droplets in these different arrays, we revealed some novel aspects of nucleation such as unique distributions of crystallized drops. During the heating step, we found that in disordered 2D emulsion arrays, destabilization is a two-step process including spontaneous coalescence events followed by coalescence propagation. By quantifying the frequency of n-body coalescence events we found that smaller droplets have limited coalescence propagation, and therefore they are more stable. We also monitored the shape evolution of multi-droplet coalescing aggregates, and characterized the restructuring and cross-linking events as the key features of partial coalescence process. Our results show that processes in emulsions, as many-body systems, are driven by collective inter-droplet interaction, which can induce the propagation of these processes within the system. Discovering these interactions elucidates the physics behind the unusual behavior of complex fluids.

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Keywords

Nucleation, Partial coalescence, Collective dynamics

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