Hydration lubrication of polyzwitterionic brushes leads to nearly friction- and adhesion-free droplet motion

Abstract

Recently, there has been much progress in the design and application of oil-repellent superoleophobic surfaces. Polyzwitterionic brush surfaces are of particular interest, because of their ability to repel oil under water, even in the absence of micro-/nano-structures. The origin of this underwater superoleophobicity is attributed to the presence of a stable water film beneath the oil droplet, but this had not been demonstrated experimentally. Here, using optical interferometric techniques, we show that an oil droplet effectively hydroplanes over a water film, whose thickness is between one hundred and hundreds of nanometres. In addition, using a custom-built Droplet Force Apparatus, we measured the friction and adhesion forces to be in the nN range for millimetric-sized droplets. These forces are much lower than for other classes of well-known liquid-repellent surfaces, including the lotus-leaf effect and lubricant-infused surfaces, where the typical force is on the order of µN. arXiv:1906.09910v2 [cond-mat.soft]

Mapping micrometer-scale wetting properties of superhydrophobic surfaces

“…underwater superleophobic surface). Droplet probe AFM technique described here will greatly complement other ultra-sensitive surface wetting characterization tools previously reported, such as the droplet force apparatus [16,17] and the scanning droplet adhesion microscope [11]. However unlike previous approaches, our approach does not require any specialized instrument beyond a conventional AFM setup, which are available to many research groups.…”

“…The most common approach is to use a cantilever force sensor to directly measure the friction and adhesion forces (with a typical 0.1 µN resolution) acting on a millimetric droplet [14][15][16]. Recently, by greatly suppressing the environmental noise, our group has improved the resolution of such an instrument (which we named the Droplet Force Apparatus) to * daniel@imre.a-star.edu.sg about 5 nN [17]. More impressively, by combining a sensitive force sensor with a motorized sample stage, Ras and co-workers were not only able to achieve a similar nN force resolution, but were also able to map wetting variations with a lateral resolution of 10 µm [11].…”

Water droplet friction and rolling dynamics on superhydrophobic surfaces

“…The friction of our etched silicon samples spans a very wide force range due to their topographical differences, where the most sparse spikes of sample A allow for an ultralow dimensionless friction force F μ =γD ¼ 4 ± 3 ð ÞÁ10 À4 that overcomes the friction of previously studied superhydrophobic surfaces made of micropillars 9,45 , SOCAL 9 , and silicone nanofilaments 19 , as well as lubricated surfaces 9 and a "nearly friction-and adhesion-free" underwater-SOCAL surface 20 (Fig. 4).…”

“…The tilt-stage 4,5,15,16 and oscillating droplet tribometer 17 techniques have been developed to measure droplet friction forces. One further, especially important emerging technique is the use of force-calibrated elastic cantilevers (spring constant k p ) to measure the friction force of droplets moving on various substrates [7][8][9][18][19][20] , including moderately superhydrophobic 7,9,19 , SOCAL 9,20 , and lubricated 8,9 coatings. The force is directly obtained through optical detection of the deflection (Δx) of the cantilever: F ¼ Àk p Δx.…”

“…The force is directly obtained through optical detection of the deflection (Δx) of the cantilever: F ¼ Àk p Δx. Various kinds of cantilevers have been used to study liquid-repellent surfaces, including thick rectangular glass capillaries (side lengths 0:04 À 0:4 mm, k p ¼ 100 À 200 nN μm −1 , force resolution~40 nN) 7,19 and thick polymeric tubes (inner/ outer radii 0:29=0:36 mm, k p ¼ 2 À 30 nN μm −1 , force resolution $ 10 À 100 nN) 8,9,20 . The main downside of those cantilevers is that their width is comparable to the droplet size, which affects the droplet shape and disturbs the flow inside the droplet.…”