Drop transfer between superhydrophobic wells using air logic control.

Superhydrophobic surfaces aid biochemical analysis by limiting sample loss. A system based on wells here tolerated tilting up to 20° and allowed air logic transfer with evidence of mixing. Conditions for intact transfer on 15 to 60 µL drops using compressed air pressure operation were also mapped.

Liquid body formation from a semispherical superhydrophobic well on a small incline.

In this work, drop formation on a slightly inclined superhydrophobic substrate with liquid at various flow rates delivered through a semispherical well was investigated. Due to the initial dry well condition in the first drop produced, the inertial force from liquid filling allowed the well's edge hysteresis to be more readily breached, in which flow rates of 16 mL/min and above could create a jet that appeared to be able to "pierce" through the top of the semispherical drop without disrupting its form and growth very much. For subsequent drops, the well's edge hysteresis at flow rates of 14 mL/min and above helped to support an "egg" like form. In contrast, this form could not be developed on a similarly inclined superhydrophobic substrate without a well. The findings here assist to establish the flow rate ranges for consistent discrete volume delivery in biochemical analysis and serves as a means to conduct investigations to better reconcile the tendency of liquids to assume drops or develop jets.

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.

Microplate well coverage mixing using superhydrophobic contact.

Two important challenges in microplate instrumentation are to achieve full well sample coverage and complete mixing. An effective approach of using superhydrophobic rods to accomplish these challenges is reported here. Experiments conducted showed that analytes above 50µl could be made to completely cover the bottom of 96-well standard and transparency microplates. Complete mixing was accomplished by moving the rod parallel to the well bottom while contacting the liquid. The approach is simple and controlled, and it minimizes the problems of spillage and cross-contamination. It works with analytes with varied volumes and of different viscosities present in each well of the microplate.