Impact of Picoliter Droplets on Superhydrophobic Surfaces with Ultralow Spreading Ratios

Abstract

The impact of picoliter-sized water droplets on superhydrophobic CF(4) plasma fluorinated polybutadiene surfaces is investigated with high-speed imaging. Variation of the surface topography by plasmachemical modification enables the dynamics of wetting to be precisely controlled. Final spreading ratios as low as 0.63 can be achieved. A comparison of the maximum spreading ratio and droplet oscillation frequencies to models described in the literature shows that both are found to be much lower than theoretically predicted.

Topographical length scales of hierarchical superhydrophobic surfaces

“…Three different types of plasmachemical textured surfaces were prepared by varying the process parameters (see Table 1). For each, the surface XPS F:C ratio remained constant [18,20]. …”

“…Contact angle hysteresis values were calculated from the difference between advancing and receding angles, which were measured by respectively increasing and decreasing the droplet size until the contact line was observed to move [40]. Our previous studies have shown that picolitre droplets result in bouncing off the CF 4 plasmachemical textured surfaces, and therefore microlitre droplets were employed in the current study [20].…”

“…The extent of this polymer chain rearrangement and etching gives rise to the observed range of surface topographies (i.e. by varying parameters such as plasma power and exposure time during plasma processing) [18,20,48]. Also, VUV irradiation penetrates below the polymer film surface [49] to cause dissociation of polymer chain σ-bonds [50], and facilitating cross-linking of the subsurface through reaction with unsaturated polybutadiene carbon-carbon double bonds to create regions more resilient towards ablation (surface roughness) [51].…”

“…One of the most highly superhydrophobic surfaces reported to date is based on the CF 4 plasmachemical functionalisation and texturing of polybutadiene to create low surface energy (fluorinated) hierarchical roughness topographical structures [18,19]. The impact and spreading of water droplets onto these surfaces has been shown to be strongly governed by the hierarchical surface topography (for similar overall surface chemical composition and roughness values) [20,21].…”

Velocity control of nanoliter droplets using a pneumatic dispensing system

“…Especially, to ensure highly accurate positioning of droplets in well-defined substrate locations, the velocity and direction of the droplets ejected from the outlet must be uniform [17]. The velocity of droplets affects how the droplet contacts the substrate (impact process) and how straightly it flies to the substrate (directionality) [18]. High velocity provides a good directionality of the dispensed droplet but causes droplet splashing during impact process, whereas low velocity induces poor directionality and reduces the precision of positioning [19]; therefore, if a low-velocity droplet is to reach a desired location on the substrate, the droplet's flight distance must be short.…”