Formation of surface nanodroplets facing a structured microchannel wall

Abstract

Surface nanodroplets are important units for lab-on-a-chip devices, compartmentalised catalytic reactions, high-resolution near-field imaging, and many others. Solvent exchange is a simple solution-based bottom-up approach for producing surface nanodroplets by displacing a good solvent of the droplet liquid by a poor one in a narrow channel in the laminar regime. The droplet size is controlled by the solution composition and the flow conditions during the solvent exchange. In this paper, we investigated the effects of local microfluidic structures on the formation of surface nanodroplets. The microstructures consist of a microgap with a well-defined geometry, embedded on the opposite microchannel wall, facing the substrate where nucleation takes place. For a given channel height, the dimensionless control parameters were the Peclet number of the flow, the ratio between the gap height and the channel height, and the aspect ratio between the gap length and the channel height. We found and explained three prominent features in the surface nanodroplet distribution at the surface opposite to the microgap: (i) enhanced volume of the droplets; (ii) asymmetry as compared to the location of the gap in the spatial droplet distribution with increasing Pe; (iii) reduced exponent of the effective scaling law of the droplet size with Pe. The droplet size also varied with the aspect and height ratios of the microgap at a given Pe value. Our simulations of the profile of oversaturation in the channel reveal that the droplet size distribution may be attributed to the local flow patterns induced by the gap. Finally, in a tapered microchannel, a gradient of surface nanodroplet size was obtained. Our work shows the potential for controlling nanodroplet size and spatial organization on a homogeneous surface in a bottom-up approach by simple microfluidic structures.

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

“…Consequently, it is expected that z max B Pe 1/2 and t B h 2 Pe À1/2 /D, reflecting that at a fixed position downstream for large advection velocity the blob of oversaturation will be smeared out less as less time has passed. 20 Thus z max Át B h 2 /D, 20 resulting in…”

“…This result demonstrates the mixing behaviour during solvent exchange is well described Taylor-Aris dispersion. [20][21][22][23]…”

“…On the other hand, for a fixed flow rate, the droplet volume scales with the channel height h with ph 3 and thus can be modulated by modulating the channel height. 20 These influences can be understood by describing the mixing of the two solutions through Taylor-Aris dispersion. [21][22][23] Consequently, for given channel dimensions, it is expected that the duration of the oversaturation pulse t scales Pe À1/2 .…”

“…[21][22][23] Consequently, for given channel dimensions, it is expected that the duration of the oversaturation pulse t scales Pe À1/2 . 20 So far, however, the growth time of droplets at different flow rates has not yet been directly measured from experiments and a quantitative correlation between the droplet growth time and the flow rate of the solvent exchange is yet to be established.…”

Bubbles nucleating on superhydrophobic micropillar arrays under flow

“…As many natural and synthetic surfaces are structured, 14,15 it is of high interest to use the potency of morphology for controlling heterogeneous nucleation. Previous studies showed the power of geometry in nucleation of droplets from vapor [16][17][18] or solutions, 19 and in shaping foams in 2D. 20 Yet, the impact of surface micromorphology and curvature on bubble nucleation under flow was not experimentally studied.…”