Surveying the Directional Transport of Water Droplets upon Impact on Metallic Topographic Gradients.

Drop impact phenomena on raw, polished, and topography-altered gradient surfaces are investigated and presented. The main aim of this study is to demonstrate that in using a one-step industrial patterning process, it is possible to obtain metal topographical wetting gradients that can produce various desired outcomes after droplet impact. The findings could be applied to improving wind or steam turbine blades. The ranges of Weber (We) and Reynolds (Re) numbers in the study are 3-300 and 650-6500, respectively. It is demonstrated that for a fixed We, the droplet transport outcomes change from bouncing-off to side-flipping to deposition depending on the impact location and the gradient strength. The effect of We in combination with the gradient strength was also considered to demonstrate droplet behavior similar to that observed on a uniform water repellent surface and on biphilic systems. In addition, full bouncing-off and directional control have been demonstrated. For the condition We = 95 ± 3, it was possible to achieve a maximum droplet recoil height of ∼6 mm and a side motion of almost 8 mm. A combination of different outcomes (e.g., splashing on one side of a droplet and passive horizontal translation on another) was observed on the studied gradients at We > 200 due to different wetting regimes across the droplet's three-phase line.

Patterning Wettability for Open-Surface Fluidic Manipulation: Fundamentals and Applications.

Effective manipulation of liquids on open surfaces without external energy input is indispensable for the advancement of point-of-care diagnostic devices. Open-surface microfluidics has the potential to benefit health care, especially in the developing world. This review highlights the prospects for harnessing capillary forces on surface-microfluidic platforms, chiefly by inducing smooth gradients or sharp steps of wettability on substrates, to elicit passive liquid transport and higher-order fluidic manipulations without off-the-chip energy sources. A broad spectrum of the recent progress in the emerging field of passive surface microfluidics is highlighted, and its promise for developing facile, low-cost, easy-to-operate microfluidic devices is discussed in light of recent applications, not only in the domain of biomedical microfluidics but also in the general areas of energy and water conservation.

Water Condensation and Droplet Shedding Behavior on Silica-Nanospring-Coated Tubes.

In this work, the water condensation performance of methylated silica-nanospring (SN)-coated horizontal aluminum tubes is assessed. Coated samples with varying nanospring mat thicknesses, from 784 to 2902 nm, were studied, which exhibited static contact angles and CA hysteresis values of 155° and 16°, respectively. Dropwise condensation and increased droplet shedding were observed on these coated tubes. Video analysis determined that tubes with 15 and 20 min SN growth times experienced an 84% increase in the condensate removal rate over the baseline. Moreover, with a hybrid wettability consisting of alternating regions of SN and bare aluminum, a 96% increase in condensate removal was experienced. Additionally, the average droplet departure size was reduced on these SN-coated tubes. SEM imaging and XEDS analysis were also performed on the tubes and revealed that the coating was reasonably durable having withstood the condensation environment. Moreover, the coated tubes were shown to exhibit the same XEDS spectra both before and after testing.

Variations of the Static Contact Angle of Ferrofluid Droplets on Solid Horizontal Surfaces in External Uniform Magnetic Fields.

The application of external uniform magnetic fields to ferrofluid droplets affects their magnetic order at the nanoscale as well as their shape at the macroscale, thus changing their contact angle with the surface. In this work, the effects of external uniform magnetic fields on the contact angles between different oil-based ferrofluid droplets and a handful of horizontal surfaces of varying wettability were studied. The contact angle is no longer constant around the ferrofluid droplet; rather, it varies in a rich yet predictable way. Droplets dispensed in the presence of the magnetic field on oleophobic surfaces adjust such that the contact angle increases at the front and back ends and decreases at the two perpendicular positions. The opposite behavior is reported for ferrofluid droplets on oleophilic surfaces. These direction-dependent changes in the contact angle can have a significant impact on the behavior of ferrofluid droplets on gradient surfaces where they can either diminish or enhance the surface tension gradient. Our work is fundamentally relevant to potential applications involving the controlled movement of ferrofluid droplets on surfaces like the lab-on-a-chip under the combined effects of a magnetic field and either a surface tension gradient or an electric field (i.e., electrowetting). It is important to understand how the two effects interact for the optimal utilization of these effects in future applications.

Characterization of Methyl-Functionalized Silica Nanosprings for Superhydrophobic and Defrosting Coatings.

Thin non-perfluoroalkoxy superhydrophobic coatings are desirable for heat exchangers because of their lower thermal resistance and reduced environmental concerns. Coatings requirements must also include robustness and longevity and facilitate high defrosting rates in refrigeration applications to warrant their adoption and use. Methyl-functionalized silica nanosprings (SN) possess water droplet static contact angles above 160° with contact angle hysteresis values as low as 6.9° for a sub-micrometer-thick coating. The methyl functional groups render the silica surface hydrophobic, whereas the geometrical and topographical characteristics of the nanosprings make it super-hydrophobic. Results show that SN are capable of removing 95% of the frost from the surface at a lower temperature than the base aluminum substrate. The sub-micrometer SN coating also decreases the time to defrost by ≈1.5 times and can withstand more than 20 frosting-defrosting cycles in a high humidity environment akin to real working conditions for heat exchangers.