Abstract
In this work, we study the spontaneous spreading of water droplets immersed in oil and report an unexpectedly slow kinetic regime not described by previous spreading models. We can quantitatively describe the observed regime crossover and spreading rate in the late kinetic regime with an analytical model considering the presence of periodic metastable states induced by nanoscale topographic features (characteristic area ~4 nm2, height ~1 nm) observed via atomic force microscopy. The analytical model proposed in this work reveals that certain combinations of droplet volume and nanoscale topographic parameters can significantly hinder or promote wetting processes such as spreading, wicking, and imbibition.
Highlights
The initial spreading dynamics for R < RC can be described by power-law scalings R ∝ tα with exponents α ≃ 2/3 to 1, which can be accounted for by using damping coefficients estimated by molecular kinetic theory (MKT) in Eq (3)
During the early dynamics of droplet spreading extending for about 0.1 s we observe power-law behaviors governed by capillary forces and effective damping forces that can be rationalized by MKT, as reported in previous studies[21,39,40]
The damping coefficient predicted by MKT solely considers the viscosity of the ambient phase, which is aobut 100 times larger than the droplet viscosity in our experiments
Summary
Power-law behaviors with an expected range of exponents (α = 0.1 to 1) are observed during the initial ~0.1 seconds of the spreading process, after which there is a crossover to an unexpectedly slow regime that persists for around 103 seconds until attaining the mechanical equilibrium condition prescribed by θE. Such metastable states correspond to local minima in the free energy profile ( ) given by Eq (2), which can only exist when the droplet is sufficiently close to equilibrium and K|R2 − RE2| ≤ Δ /Ad. the kinetic spreading regime governed by Eq (6) should be only be observed for contact radii R > RC larger than the crossover radius
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