Drops
with larger volumes placed over a superhydrophobic (SH) surface with a hole do not fall through unless
they are evaporated to a size that is small enough. This feature offers
the ability to preconcentrate samples for biochemical analysis. In
this work, the influence of pinning on the behavior of drops placed
on a 0.1 mm thick SH substrate with a 2 mm diameter hole as they evaporated
was investigated. With 16 μL of water dispensed, the sessile
drop component volume was initially higher than that of the overhanging
drop component and maintained this until the later stages where almost
identical shapes were attained and full evaporation was achieved without
falling off the hole. With 15 μL of water dispensed, the volume
of the sessile drop was initially higher than that of the overhanging
drop component but the liquid body was able to squeeze through the
hole after 180 s due to the contact line not having sufficient pinning
strength when it encountered the edge of the hole. This resulted in
the liquid body either falling through the hole or remaining pinned
with an oval-like shape. When it did not fall-off, the liquid body
had volume and contact angle characteristics for the sessile drop
and overhanging drop components that were reversed. In the later stages,
however, nearly identical shapes were again attained and full evaporation
was achieved without falling off the hole. The effects of pinning,
despite the substrate being SH, offer another path toward achieving
practical outcomes with liquid bodies without the need for chemical
surface functionalization. Similarities and differences could be seen in the behavior of
a sessile drop on a SH plate that was inclined at 30° to the
horizontal and evaporated.
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.
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