Water drop impacts on regular micropillar arrays: Asymmetric spreading

Abstract

Eye-catching shapes are produced when water drops land vertically and spread on horizontal surfaces with micropillars arranged in regular square arrays. The positions of protrusions and fingers are often determined by the microstructure design and may be produced repeatably, which suggests possible manufacturing and analytical applications. This paper uses high-speed imaging of droplet shapes following impact to record and analyze asymmetries as drop spreading reaches its maximum extent. The range of experimental parameters used produced results varying (often non-monotonically) from symmetric spreading to many fingers. Impact Weber numbers (We) were systematically adjusted between 50 and 250, while surface microstructures featured circular (◯) and square (□) cross-sectional pillars of width d = 20 μm; height h = 15, 22, or 30 μm; and pitch p = 40, 60, or 80 μm. Many observed trends correlate with the extent of the fully wet impact region, including a general increase in asymmetry with We, p, and for □ rather than ◯ pillars. More detailed understanding of asymmetry mechanisms is also developed. For example, protrusions may be nucleated by jetting in directions of high gas flow within 100 μs of impact. A new analysis of gas flow under the drop, which accounts for Laplace pressure, explains anomalous spreading and asymmetry measurements. Reduced spreading velocity is identified as the cause of finger suppression where the microstructure is wet.

Influence of rheology and micropatterns on spreading, retraction and fingering of an impacting drop

“…However there are no clear differences during the spreading phase for the various micropillar array surfaces. These observations are similar to those in a recent study of water drop impacts on similar surfaces, 40 which noted that the effect of different microstructure designs may be limited because surfaces which produce relatively high viscous losses for fluid flowing through the microstructure also tend to have smaller areas of microstructure that are completely wet by the droplet. In previous work using 0.2–1% carbopol solutions, differences between spreading on microstructured and wetting glass surfaces were only observed at We ≈ 1000.…”

“…Fingers have recently been experimentally linked to gas flow via observations of jets which nucleate protrusions from the spreading lamella. 40 The quantity h ( p – d ), known as the ‘corridor area’ between the rows of posts from a horizontal line of sight, provides a measure of how easily fluids can move through a microstructure in on-axis directions. The corridor area and other aspects of the geometry of the p = 40 μm pillar arrays can be quantified using the parameters included in Table 2.…”

“…One of the most interesting observations for water drop impacts on micropillar patterns has been spreading which forms shapes determined by the pattern. 33,34,38–40 Mostly these shapes are formed by fingers emerging from the rim of the spreading lamella. On square micropillar arrays the fingers typically occur on-axis and at 45° to those directions.…”

“…The occurrence of fingers increases with impact velocity, and depends critically on the array design. 33,39,40 The geometry of microstructure penetration, 34 which is known to affect droplet bouncing on superhydrophobic surfaces, 41 plays an important role in the eventual shape of the drop. Gas escaping from under a landing drop affects spreading, 35 and on micropatterns the link between paths of least resistance for gas escape and spreading asymmetries has been postulated 39 and studied.…”

“…Gas escaping from under a landing drop affects spreading, 35 and on micropatterns the link between paths of least resistance for gas escape and spreading asymmetries has been postulated 39 and studied. 33,40…”