Abstract
AbstractBubbles attached to surfaces are ubiquitous in nature and in industry. However, bubbles are problematic in important technologies, including causing damage to the operation of microfluidic devices and being parasitic during heat transfer processes, so considerable efforts have been made to develop mechanical and electrical methods to remove bubbles from surfaces. In this work, liquid dielectrophoresis is used to force a captive air bubble to detach away from an inverted solid surface and, crucially, the detached bubble is then held stationary in place below the surface at a distance controlled by the voltage. In this “levitated” state, the bubble is separated from the surface by liquid layer with a voltage‐selected thickness at which the dielectrophoresis force exactly counterbalances the gravitational buoyancy force. The techniques described here provide exceptional command over repeatable cycles of bubble detachment, levitation, and reattachment. A theoretical analysis is presented that explains the observed detachment–reattachment hysteresis in which bubble levitation is maintained with voltages in an order of magnitude lower than those used to create detachment. The precision surface bubble removal and control concepts are relevant to situations such as nucleate boiling and microgravity environments and offer an approach toward “wall‐less” bubble microfluidic devices.
Highlights
Introduction such systemsPassive mechanical mechanisms typically provide management of already free bubbles, rather than activelyIn microfluidic systems, undesirable bubbles can often be dislodging sessile or captive bubbles.[8]
We have shown how a vapor filled bubble can be “dewetted” and detached from a solid surface by dielectrowetting with an applied voltage VD which exceeds a critical value
Once the bubble is free from an inverted surface, it enters a state of voltage-controlled levitation where changing the voltage allows precise, micrometer scale control over the separation distance
Summary
While bubbles can be detached in this manner via dielectrowetting for any orientation of the solid surface, if the surface is immersed in an inverted orientation liquid dielectrophoretic forces can counterbalance the buoyancy forces that would otherwise drive bubbles to attach to the surface, i.e., forcing a state of bubble levitation on an intervening liquid layer of thickness h. This is demonstrated in Movie M3† in the Supporting Information where a fast rising bubble is prevented from attaching to a surface, and in Movie M4† in the Supporting Information where even a bubble that is mechanically squeezed to attempt to force it onto the surface is www.advmatinterfaces.de unable to attach to the surface, in both cases due to the surfacelocalized dielectrophoretic forces prevent bubble attachment while the voltage is applied
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