Growth dynamics of surface nanodroplets during solvent exchange at varying flow rates

Abstract

Solvent exchange is a simple solution-based process to produce surface nanodroplets over a large area. The final size of the droplets is determined by both the flow and solution conditions for a given substrate. In this work, we investigate the growth dynamics of surface nanodroplets during solvent exchange by using total internal reflection fluorescence microscopy (TIRF). The results show that during the solvent exchange, the formation of surface nanodroplets advanced on the surface in the direction of the flow. The time for the number density and surface coverage of the droplets to reach their respective plateau values is determined by the flow rate. From the observed evolution of the droplet volume and of the size of individual growing droplets, we are able to determine that the growth time of the droplets scales with the Peclet number Pe with a power law ∝Pe-1/2. This is consistent with Taylor-Aris dispersion, shedding light on the diffusive growth dynamics during the solvent exchange. Further, the spatial rearrangement of the droplets during coalescence demonstrates a preference in position shift based on size inequality, namely, the coalesced droplet resides closer to the larger of the two parent droplets. These findings provide a valuable insight toward controlling droplet size and spatial distribution.

Formation of Surface Protic Ionic Liquid Nanodroplets for Nanofabrication

“…This increase in contact angle (CA) during growth is contrary to the typical formation of water and other organic oils where the surface droplet growth process on a homogeneous substrate is always at a constant contact angle. [35,36] However, this dynamic CA process is consistent with the dewetting process observed with amphiphiles, and shares an interesting parallel with previous simulations which revealed that ionic liquid droplets increased in contact angle as the droplet increased in size. [28] The droplet dewetting phenomenon observed in the whole droplet growth process of about 60 s resembles the autophobing observed for droplets of amphiphile solutions on a substrate, where structuring of the amphiphile at the substrate induces a spontaneous dewetting.…”

“…On the other hand, the mean diameter dramatically increased to the maximum value of ≈3 µm in ≈10 s and then gradually reduced and stabilized at ≈2.6 µm at 100 s. The surface coverage illustrated a similar trend: increasing to 38% in the first 10 s and decreasing back to 23% at the end. The reduction in surface coverage was more pronounced than expected for a continual nucleation and growth, which is limited by the competition and continual coalescence between droplets . To further examine the dynamic growth process of an individual droplet, which is identified by a yellow arrow in Figure , has been tracked.…”

“…The EAN droplets transfer to a spherical‐cap shape, driven by the surface energy relationship amongst EAN, cyclohexane, and the glass substrate according to Young's equation. This increase in contact angle (CA) during growth is contrary to the typical formation of water and other organic oils where the surface droplet growth process on a homogeneous substrate is always at a constant contact angle . However, this dynamic CA process is consistent with the dewetting process observed with amphiphiles, and shares an interesting parallel with previous simulations which revealed that ionic liquid droplets increased in contact angle as the droplet increased in size …”

“…The reduction in surface coverage was more pronounced than expected for a continual nucleation and growth, which is limited by the competition and continual coalescence between droplets. [34] To further examine the dynamic growth Adv. Mater.…”

Growth dynamics of microbubbles on microcavity arrays by solvent exchange: Experiments and numerical simulations

“…For the formation of surface nanodroplets, recent work has shown that the flow rate of the solvent exchange is one of the most important parameters that influences the growth dynamics of the droplets [34]. Both theoretical analysis and experimental measurements by total internal reflection microscope confirm that the growth time scales with the square root of the Peclect number (the dimensionless flow rate) of the displacing solvent [35]. The flow rate therefore has significant influence on the size and number density of the droplets.…”