Abstract
The control of liquid motion on the micrometer scale is important for many liquid transport and biomedical applications. An efficient way to trigger liquid motion is by introducing surface tension gradients on free liquid interfaces leading to the Marangoni effect. However, a pronounced Marangoni-driven flow generally only occurs at a liquid–air or liquid–liquid interface but not at solid–liquid interfaces. Using superhydrophobic surfaces, the liquid phase stays in the Cassie state (where liquid is only in contact with the tips of the rough surface structure and air is enclosed in the indentations of the roughness) and hence provides the necessary liquid–air interface to trigger evident Marangoni flows. We use light to asymmetrically heat this interface and thereby control liquid motion near superhydrophobic surfaces. By laser scanning confocal microscopy, we determine the velocity distribution evolving through optical excitation. We show that Marangoni flow can be induced optically at structured, air-entrapping superhydrophobic surfaces. Furthermore, by comparison with numerical modeling, we demonstrate that in addition to the Marangoni flow, buoyancy-driven flow occurs. This effect has so far been neglected in similar approaches and models of thermocapillary driven flow at superhydrophobic surfaces. Our work yields insight into the physics of Marangoni flow and can help in designing new contactless, light-driven liquid transport systems, e.g., for liquid pumping or in microfluidic devices.
Highlights
Controlled liquid transport, especially guiding liquid flows on the microscale, is crucial for numerous applications, such as microfluidics for chemical or biochemical analysis or inkjet printing techniques
We show that Marangoni flow can be induced optically at structured, air-entrapping superhydrophobic surfaces
Further conclusions can be drawn from the Marangoni number Ma, which characterizes the strength of Marangoni flow
Summary
Controlled liquid transport, especially guiding liquid flows on the microscale, is crucial for numerous applications, such as microfluidics for chemical or biochemical analysis or inkjet printing techniques. The Marangoni effect has been reported to induce directional/convective flows in the liquid phase,[2−5] to guide droplets or floating objects at fluid interfaces,[6−9] and to determine various spreading dynamics phenomena, e.g., fingering instabilities,10,11 “Marangoni bursting”.12 In addition to these observations, the Marangoni effect has been reported as a promising approach for the actuation of droplets or liquid films in microfluidic systems,[10,13−20] e.g., for directional transport of droplets on a nonuniformly heated solid surface[16,17,21] or when a droplet is immersed in another immiscible liquid which is imposed with a temperature gradient.[22−27] The Marangoni effect has been reported to induce directional/convective flows in the liquid phase,[2−5] to guide droplets or floating objects at fluid interfaces,[6−9] and to determine various spreading dynamics phenomena, e.g., fingering instabilities,10,11 “Marangoni bursting”.12 In addition to these observations, the Marangoni effect has been reported as a promising approach for the actuation of droplets or liquid films in microfluidic systems,[10,13−20] e.g., for directional transport of droplets on a nonuniformly heated solid surface[16,17,21] or when a droplet is immersed in another immiscible liquid which is imposed with a temperature gradient.[22−27]
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