Manipulation of nano-liter droplets is a key step for many emerging technologies including ultra-compact microfluidics devices, 3D and flexible electronic printing. Despite progress, contamination-free generation and release of droplets by compact low-cost devices remains elusive. Here, inspired by the butterflies’ minute manipulation of fluids, we have engineered a superamphiphobic bionic proboscis (SAP) layout that surpasses synthetic and natural designs.1 We demonstrate the scalable fabrication of SAPs with tunable inner diameters down to 50 µm by the rapid gas-phase nanotexturing2 of the outer and inner surface of readily available hypodermic needles. Optimized SAPs achieve contamination-free manipulation of water and oil droplets down to a volume of 10 nl and a liquid surface tension of 26.56 mN/m in line good agreement our theoretical predictions3. The unique potential of this layout is showcased by the rapid and carefully controlled in-air synthesis of core-shell droplets with well-controlled composition. These findings provide a new low-cost tool for high-precision manipulation of nano-liter droplets, offering a powerful alternative to established thermal- and electrodynamic-based devices.
Figure 1 shows an exemplary superamphiphobic nanocoating patterned on an Australian coin and on a glass substrate. This is obtained by the deposition of nanoparticles aerosols generated either by low-temperature liquid or high temperature flame sprays. The resulting coatings can be made robust by the predisposition of a supporting micro-nanotextured layer of interpenetrated polymer networks.4
Figure 2a shows a schematic of the SAPs’ layout. Conformal coatings of re-entrant nanotextures are rapidly fabricated on the inner and outer walls of hypodermic needles by aerosol deposition of SiO2 nanoparticles. This results in the self-assembly of an ultraporous (98%) nanoparticle network that is, thereafter, functionalized by atmospheric chemical vapour deposition of low surface energy fluoro-groups. The inner wall functionalization prevent in-needle adhesion, enabling superior control over droplet release and size, while resisting contamination by capillary rise (Figure 1b,c). The need to break capillary-bridges for droplet detachment is also prevented, further reducing the risk of contaminating the inner walls with liquid residues. The outer surface functionalization prevents contamination by creep during droplet generation and immersion in liquids. The performance of this design was initially demonstrated by the generation of a 400 nL oil (tetradecane) droplet with a SAP and a bare needle used as comparison. While the bare needle suffered from visible oil creep on its exteriors, the SAP preserved a pristine surface with no contamination (Figure 1b). Figure 3 demonstrate the facile contamination free generation and handling of core-shell nanodroplets by this SAP design.
REFERENCES:
[1] W. S. Wong, G. Liu and A. Tricoli, Superamphiphobic Bionic Proboscis for Contamination-Free Manipulation of Nano and Core-Shell Droplets. Small 2017, 10.1002/smll.201603688.
[2] G. Liu, W. S. Wong, N. Nasiri and A. Tricoli, Ultraporous superhydrophobic gas-permeable nano-layers by scalable solvent-free one-step self-assembly. Nanoscale 2016, 8 (11), 6085-6093.
[3] W. S. Y. Wong, G. Liu, N. Nasiri, C. Hao, Z. Wang and A. Tricoli, Omnidirectional Self-Assembly of Transparent Superoleophobic Nanotextures. ACS Nano 2017, 11 (1), 587-596.
[4] W. S. Wong, Z. H. Stachurski, D. R. Nisbet and A. Tricoli, Ultra-durable and transparent self-cleaning surfaces by large-scale self-assembly of hierarchical interpenetrated polymer networks. ACS applied materials & interfaces 2016, 8 (21), 13615-13623.
