Self-driven surface micromixers (SDSM) relying on patterned-wettability technology provide an elegant solution for low-cost, point-of-care (POC) devices and lab-on-a-chip (LOC) applications. We present a SDSM fabricated by strategically patterning three wettable wedge-shaped tracks onto a nonwettable, flat surface. This SDSM operates by harnessing the wettability contrast and the geometry of the patterns to promote mixing of small liquid volumes (µL droplets) through a combination of coalescence and Laplace pressure-driven flow. Liquid droplets dispensed on two juxtaposed branches are transported to a coalescence station, where they merge after the accumulated volumes exceed a threshold. Further mixing occurs during capillary-driven, advective transport of the combined liquid over the third wettable track. Planar, non-wettable "islands" of different shapes are also laid on this third track to alter the flow in such a way that mixing is augmented. Several SDSM designs, each with a unique combination of island shapes and positions, are tested, providing a greater understanding of the different mixing regimes on these surfaces. The study offers design insights for developing low-cost surface microfluidic mixing devices on open substrates.Microfluidic devices capable of achieving complex liquid-handling tasks, such as transport, metering, separation and mixing, have found niche applications in numerous fields, including point-of-care (POC) diagnostics 1 , lab-on-a-chip (LOC) applications 2-5 , and micro total analysis systems (μTAS) 6 . Although most conventional microfluidics devices have historically deployed flow-through systems, a more recent strategy of handling liquid samples in the form of discrete droplets has gained popularity 7 . Droplet-based microfluidics is advantageous over flow-through microfluidics, since the liquid sample handling in the former is relatively free from the common problems of the latter, such as axial dispersion, sample dilution, and cross-contamination 8, 9 . In droplet-based microfluidics, individual (or sequences of) droplets of the sample liquid may be handled either within an immiscible liquid in a closed microchannel 10 or on open surfaces 11 . The dispensed liquid volumes range from 1 μL to 1 mL; the lower range is common to many microfluidic applications 12 , while the larger volumes are relevant to on-chip liquid storage 13 , or some specialized microfluidic applications that require larger samples (e.g. whole-blood assays 14,15 ). Open-surface type microfluidic devices offer the possibility of low-cost fabrication -these devices can be built on low-cost, paper or plastic surfaces and do not require elaborate fabrication of embedded microchannels -and hence, are ideally suited for POC diagnostics 16 .Like the flow-through microfluidic devices, rapid and efficient mixing is also an essential pre-requisite for open-surface microfluidic platforms. Open-surface micromixers may be of active or passive type, depending on whether external energy input is required or not, respectivel...
Eco-friendly, water-repellent coatings made by combining lycopodium spores and a natural wax.
pollution. Repellent surfaces hinder liquid spreading and for that purpose, are frequently used for fluid management on open surfaces. In some cases, both repellent and adhesive domains are required for efficient fluid management. [1] Wettability engineering, i.e., the spatial modification of surface energy and morphology, is a suitable option for fluid management and has been explored extensively to deliver innovative advances in materials chemistry, fabrication techniques, and a fundamental understanding of tunable liquid properties from extreme repellency to extreme wetting. In particular, superliquid-repellent surfaces have shown great promise for self-cleaning, [2] anti-fogging, [3] anti-icing, [4] drag reduction, [5] and corrosion resistance [6] applications, among others. However, the majority of prior research has focused on applications employing high-surface-tension (γ) liquids (e.g., water, γ water ≈ 72 mN m −1 ). Difficulties arise when designing surfaces that are super-repellent to low-surface-tension liquids (20-40 mN m −1 ), where wetting interactions are more sensitive to surface chemistry and surface roughness. The methodology to making a surface repellent to these liquids is essentially the same, with surface energy and surface roughness being key factors. [7] As wetting behavior is governed by competing surface forces at the contact line, [8] liquid affinity (assuming chemical and physical attributes remain constant) is mainly dependent on the probe liquid's surface tension. Ideally, high-performing superoleophobic (extreme repellency to oils and other hydrocarbons) surfaces will exhibit Cassie-Baxter [9] type wetting across a wide range of surface-tension values. However, there may be instances where Wenzel [10] type (or complete) wetting is observed with liquids of low surface tension.In order to make surfaces with specially designed reentrant structures that promote repellency to low γ liquids, [11] researchers have typically relied on costly and complex microfabrication techniques. However, scalable, large-area, sprayable superoleophobic coatings have been explored by only a few groups. [12] In addition, an ultra-omniphobic (repellent to many liquids) material (FD-POSS; (1H,1H,2H,2H-heptadec afluorodecyl) 8 Si 8 O 12 ) has been synthesized by the Air Force Research Laboratory, and is currently considered the lowest surface-energy crystalline solid material with a solid-air surface energy of 10 mN m −1 . [13] This compound is functionalized Preparing surfaces that repel low-surface-tension liquids, such as oils and hydrocarbon fuels with surface tensions below 30 mN m −1 , poses more challenges than attaining water repellency. Oleophobic surfaces are needed when organic fluids must be contained to avoid pollutant spreading. A composite material system is presented comprised of fluorinated silica (filler), a perfluoroalkyl methacrylate copolymer (binder), and fluorinated polyhedral oligomeric silsesquioxane (additive; considered the lowest surface-energy material to date), which can be ...
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