A parameter to probe microdroplet dynamics and crystal nucleation

International audienceWe present a simple and efficient digital-image processing method to simultaneously monitor the contraction of a statistically relevant number of microdroplets of the same size and the nucleation of single salt crystals inside. Each individual microdroplet image is reduced to a scalar, standard deviation sigma, and overall microdroplet dynamics is monitored using standard-deviation time-evolution plots. It is shown that this approach makes it possible to measure the nucleation time and also that microdroplets interact via water diffusion dynamics. This effect actually decreases the nucleation rate, contrary to previous findings. This " sigma approach " can be compared to recording the order parameter in phase transition, which makes it ideal for studying dynamics of systems where images are the primary outputs

Addressing the stochasticity of nucleation: Practical approaches

International audienceThis chapter presents different practical ways to address nucleation stochasticity. The methods use either statistical studies on spontaneous nucleation or local control of nucleation. Techniques developed in our laboratory are described: droplet-based microfluidics, micro-injectors in oil, external electrical or mechanical fields in confined systems. Results of nucleation kinetics obtained on various molecules, are presented in terms of metastable zone, critical supersaturation, nucleation rate, induction time, interfacial energy of the critical nucleus. These practical approaches show considerable potential to increase understanding and control of the nucleation mechanism

Monitoring Picoliter Sessile Microdroplet Dynamics Shows That Size Does Not Matter

We monitor the dissolution of arrayed picoliter-size sessile microdroplets of the aqueous phase in oil, generated using a recently developed fluidic device. Initial pinning of the microdroplet perimeter leads to a nearly constant contact diameter, thus contraction proceeds via microdroplet (micrometer-diameter) height and contact angle reductions. This confirms that picoliter microdroplets contraction or dissolution due to the selective diffusion of water in oil has comparable dynamics with microliter droplet evaporation in air. We observe a constant microdroplet dissolution rate in different aqueous solutions. The application of this simple model to solvent-diffusion-driven crystallization experiments in confined volumes, for instance, would allow us to determine precisely the concentration in the microdroplet during an experiment and particularly at nucleation