Comparisons of liquid and gaseous microdrops deposited on surfaces via a retreating tip.

The rupture of a liquid bridge has many applications while the rupture of a gaseous bridge is gaining importance in the use of bubbles to affect the speed of liquid flow over surfaces. Here, comparative experiments were conducted for liquid and gaseous bridges dispensed at fixed volumes of 6 μL on silicone (hydrophobic) and silane coated glass (hydrophilic) surfaces and with the dispensing tip retracted at different speeds. With the liquid bridge, increasing the retracting speed left behind lower volumes on the substrate. The pinch off position and the contact line radius were factors that determined the volume. The bridge first entered into a receding state before being able to restore toward equilibrium in a relaxation process closer to rupture. On silicone the contact angle was able to undergo higher degrees of hysteresis with faster tip retraction speeds due to the lower free surface energy. With gaseous bridges, only a very small volume was left behind on the silane coated glass while the volume deposited on silicone could be tuned from almost none at low retraction speeds to virtually the entire gaseous volume bridge at high retraction speeds. The tip and neck distances from the substrate increased with tip speed until 0.5 mm/s on the silicone surface, but, beyond that, the position remained invariant until rupture. With the progress toward rupture for the gaseous bridge, the contact angle advanced rather than receded and there was no relaxation stage that brought the contact angle back toward equilibrium before rupture. Overall the gaseous bridges responded very differently to tip retraction than the liquid bridges.