Abstract
When two liquids with different surface tensions come into contact, the liquid with lower surface tension spreads over the other liquid. This Marangoni-driven spreading has been studied for various geometries and surfactants, but the dynamics of miscible liquids in the binary geometry (drop-drop) has hardly been investigated. Here we use stroboscopic illumination by nanosecond laser pulses to temporally resolve the distance L(t) over which a low-surface-tension drop spreads over a miscible high-surface-tension drop. L(t) is measured as a function of time, t, for various surface tension differences between the liquids and for various viscosities, revealing a power-law L(t) ∼ tα with a spreading exponent α ≈ 0.75. This value is consistent with previous results for viscosity-limited spreading over a deep bath. The universal power law L[combining tilde] ∝ t[combining tilde]3/4 that describes the dimensionless distance L[combining tilde] as a function of the dimensionless time t[combining tilde] reasonably captures our experiments, as well as previous experiments for different geometries, miscibilities, and surface tension modifiers (solvents and surfactants). The range of this power law remarkably covers ten orders of magnitude in dimensionless time. This result enables engineering of drop encapsulation for various liquid-liquid systems.
Highlights
Liquids of low surface tension spread over liquid with high surface tension, which is known as Marangoni spreading
Drops can be encapsulated by gentle deposition onto a bath with a lower surface tension, but here the film dynamics were only assessed during coalescence[25] or for surface tension induced necking,[26,27] rather than encapsulation
To create the binary drop geometry, two pendant drops were dispensed from Teflon needles (Hamilton Company)
Summary
Liquids of low surface tension spread over liquid with high surface tension, which is known as Marangoni spreading. S E Ds represents the spreading parameter for liquid pairs in air, and Ds = s1 À s2 is the surface tension difference between the liquids These values follow from balancing the surface tension gradient with dissipation in the viscous boundary layer that develops while spreading on a deep layer.[4,5,38,39,40] Experiments performed for immiscible, non-evaporative liquids,[41,42] immiscible surfactant solutions,[43] liquid spreading over a liquid covered with insoluble surfactants,[44] and for immiscible micro-drops spreading over free-flowing thin films[32] validated this scaling argument.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
