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Evaporation of a solution dissolved chemicals concentrate until they begin to form a crystal through nucleation. Scientists studying nucleation often use microscopic droplets as miniature experiments that can run quickly in parallel, and in a small space. However, these experiments require high-resolution images, limiting the number of droplet images that can be simultaneously processed.
Researchers recently overcame this resolution challenge by focusing their measurements on the contrast between droplets and their surrounding medium. Crystal nucleation is an inherently stochastic process, and estimating each nucleation time requires growth models that work backward from the time at which the crystal grew to a detectable size. This temporal gap can range from several minutes to hours.
To determine the time it takes for a crystal to nucleate in a microdroplet, researchers generated a grid of identical salt water microdroplets covered in a thin layer of oil. Water is slightly soluble in oil at these ratios, so the water began to diffuse into its surrounding, mimicking the evaporation process.
The researchers converted the image of each droplet and its surrounding region into a scalar, the standard deviation of the pixels’ greyness, and tracked this value as it changed. When the crystal finally forms, its presence hinders the smooth evolution of the refractive index, which appears as a sudden jump in the greyness level. This enables the researchers to accurately measure the time to nucleation without resolving the crystal or making assumptions about the nucleation mechanisms.
High salt concentrations in the microdroplets cause explosive growth, cutting the delay between nucleation and detection to 0.5 seconds or less. Each droplet also disappears for a short period when its refractive index coincides with the surrounding medium.