The occurrence of concentration and temperature gradients in saline microdroplets evaporating directly in air makes them unsuitable for nucleation studies where homogeneous composition is required. This can be addressed by immersing the droplet in oil under regulated humidity and reducing the volume to the picoliter range. However, the evaporation dynamics of such a system is not well understood. In this work, we present evaporation models applicable for arrays of sessile microdroplets with dissolved solute submerged in a thin layer of oil. Our model accounts for the variable diffusion distance due to the presence of the oil film separating the droplet and air, the variation of the solution density and water activity due to the evolving solute concentration, and the diffusive interaction between neighboring droplets. Our model shows excellent agreement with experimental data for both pure water and NaCl solution. With this model, we demonstrate that assuming a constant evaporation rate and neglecting the diffusive interactions can lead to severe inaccuracies in the measurement of droplet concentration, particularly during nucleation experiments. Given the significance of droplet evaporation in a wide array of scientific and industrial applications, the models and insights presented herein would be of great value to many fields of interest.

中文翻译：

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固着盐分微滴在油中的蒸发动力学

直接在空气中蒸发的盐水微滴中存在浓度和温度梯度，这使得它们不适用于需要均匀成分的成核研究。这可以通过在调节湿度下将液滴浸入油中并将体积减小到皮升范围来解决。然而，这种系统的蒸发动力学还不是很清楚。在这项工作中，我们提出了适用于溶解溶质浸没在薄油层中的固着微滴阵列的蒸发模型。我们的模型考虑了由于分离液滴和空气的油膜的存在而导致的可变扩散距离、由于不断变化的溶质浓度而导致的溶液密度和水活度的变化以及相邻液滴之间的扩散相互作用。我们的模型与纯水和 NaCl 溶液的实验数据非常吻合。使用该模型，我们证明假设蒸发速率恒定并忽略扩散相互作用会导致液滴浓度测量严重不准确，特别是在成核实验期间。鉴于液滴蒸发在广泛的科学和工业应用中的重要性，本文提出的模型和见解将对许多感兴趣的领域具有重要价值。