Evaporation Dynamics of Sessile Saline Microdroplets in Oil

Abstract

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.

Universality of Hair as a Nucleant: Exploring the Effects of Surface Chemistry and Topography

“…The exposed hair surface resulting from grinding comprises a highly variable mixture of inner and outer regions, which is not representative of the native surface. , We therefore developed a novel strategy in which individual intact hairs were used to trigger the nucleation of KNO 3 crystals within supersaturated microdroplets. Arrays of microdroplets are ideally suited to nucleation studies since multiple experiments can be rapidly conducted while excluding the majority of heterogeneous contaminants. , High supersaturations are achievable in contaminant-free microdroplets, enabling nucleation to be triggered at will by introducing a nucleant. , We exploit this to compare the nucleant behavior over a wide concentration range.…”

“…The degree of keratin oxidation was evaluated from the sulfur 2p signal (Figure e, Section S4.3), where sulfur can exist in multiple oxidation states in biological systems . Thioester (oxidation state −2) and disulfide (−1) groups exhibit overlapping peaks centered at 164.4 eV, whereas sulfonate (+4) groups exhibit a peak at 168.6 eV .…”

Microdroplet Approach for Measuring Aqueous Solubility and Nucleation Kinetics of a Metastable Polymorph: The Case of KDP Phase IV

“…The dissolution behavior of the metastable crystal when brought in contact with the supersaturated microdroplet is probed by lateral imaging (b) Schematic diagram of the interfacial energy measurement in which the probability distribution of nucleation times is obtained from hundreds of droplets via image analysis . (Figure adapted from ref ).…”

“…Once the desired number of microdroplets is generated, the RH is lowered to 10%, causing the droplets to evaporate and eventually nucleate. Using our tailor-made evaporation model, the supersaturation ratio of the microdroplets can be obtained as a function of time. Then, we employed the inhomogeneous Poisson distribution coupled with classical nucleation theory to extract the interfacial energy from the probability distribution of nucleation times, which can be written as P ( t ) = 1 − exp [ − prefix∫ t sat t nuc J ( t ) V ( t ) normald t ] with P ( t ) as the fraction of microdroplets nucleated after time t , V ( t ) droplet volume at time t , t sat time at which the microdroplet becomes saturated, and t nuc nucleation time.…”

“…Once the desired number of microdroplets is generated, the RH is lowered to 10%, causing the droplets to evaporate and eventually nucleate. Using our tailor-made evaporation model, 10 the supersaturation ratio of the microdroplets can be obtained as a function of time. Then, we employed the inhomogeneous Poisson distribution coupled with classical nucleation theory to extract the interfacial energy from the probability distribution of nucleation times, which can be written as…”