A parameter to probe microdroplet dynamics and crystal nucleation

We 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 σ of the grey-level of pixels inside a region of interest containing the microdroplet image, 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 “σ 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.