Gas-Phase Induced Marangoni Flow Causes Unstable Drop Merging

Abstract

The merging of drops plays a key role in many processes from simple rain to complex coating applications. In technical applications, often liquids with different surface tensions merge on a substrate like inkjet printing. For a suitable set of surface tensions, one drop can form a stable wetting film that is covering the other drop. Here, we explore the dynamics of driven wetting films and show a route toward their instability when these wetting films are driven by an external source of energy, which is Marangoni stress in our case. The wetting becomes unstable via a fingering instability and can be observed in various liquid combinations. The vapor of the liquid with the lower surface tension induces a Marangoni driven flow inside the other drop that pulls the wetting film. The concentration of the driving vapor can be controlled through the spreading velocity of the corresponding drop. We use this dependence to map out the characteristics of the instability. For very high or very low spreading velocities, no instability is observed. This is summarized in a stability diagram, which has three different regimes. A detailed analysis reveals a strong coupling of the characteristics of the fingering instability to the spreading velocity. The use of the spreading velocity as a control parameter is justified by a simplified 1D model that motivates how the spreading velocity controls the concentration profile of the second liquid vapor before and at contact. The strength of the Marangoni flow that drives the instability depends on the exposure time of the sitting drop to the vapor concentration profile around the spreading drop.

Generation and Observation of Long-Lasting and Self-Sustaining Marangoni Flow

“…11 In evaporating systems, the gas and liquid phases often mutually affect each other. Complex mixing phenomena like fingering instabilities during the merging of miscible droplets 12 or the dewetting behavior of thin and thick films 13 crucially depend on the interplay of evaporation and (re-)condensation effects. Gas-phase-induced or -mediated Marangoni flows also facilitate "contactless" interaction between different liquid phases, enabling, for instance, Moses-like cleaving of droplets 14 or control of droplet motility and position.…”

Capillary filling in drop merging: dynamics of the four-phase contact point

“…[11]. In the drop merging, Marangoni tensions and the induced flow play essential roles [9,10] and can dominate this process. However, one important geometric feature of two non-identical drops on a substrate seems to be overlooked so far.…”

“…The unbalanced capillary force that drives spreading differs on these two interfaces. Additionally, the contact between the two liquid induces Marangoni tensions that can induce flow in the first drop [10]. ii) The geometry changes during the spreading of the second drop, because the first drop deforms during the spreading process.…”