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Dynamic Shaping of Femtoliter Dew Droplets. Marco Faustini, Andrea Cattoni, Jennifer Peron, Cédric Boissière, Paul Ebrard, Annie Malchère, Philippe Steyer, and David Grosso ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b07699 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018
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ACS Nano
Dynamic Shaping of Femtoliter Dew Droplets. Marco Faustini,*1 Andrea Cattoni,2 Jennifer Peron,3 Cédric Boissière,1 Paul Ebrard,1 Annie Malchère,4 Philippe Steyer,4 David Grosso*5
1 Sorbonne Université, CNRS, Collège de France, UMR 7574, Chimie de la Matière Condensée de Paris, F-75005, Paris, France 2 Centre de Nanosciences et de Nanotechnologies, CNRS, Université Paris Sud, Université Paris-Saclay, C2N−Marcoussis, 91460 Marcoussis, France 3 Univ. Paris Diderot, Sorbonne Paris Cite, ITODYS, CNRS UMR 7086, 15 rue J.-A. de Baif, 75205 Paris Cedex 13, France. 4 Matériaux Ingénierie et Science, UMR CNRS 5510, INSA de Lyon, Université de Lyon, F- 69621, Villeurbanne, France 5 IM2NP, Faculté des Sciences et Techniques, Campus de Saint Jérôme, Avenue Escadrille Normandie Niemen, 13397 Marseille, France
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KEYWORDS : dew, condensation, shaping, hydrophilic, anisotropy, nanoimprinting
ABSTRACT: Herein we show that wetting properties such as Giant Wetting Anisotropy and dynamic shaping can be observed when femtoliter (sub-micron scale) dew droplets are condensed on nanopatterned midly hydrophilic surfaces. Large scale, optically transparent, nanopatterned TiO2 surfaces were fabricated by direct nano-imprinting lithography of sol-gel derived films. Square, infinitely elongated, or circular droplets were obtained with square, line, or concentric patterns, respectively and were visualised in situ during formation and recession using optical microscopy and Environmental Scanning Electronic Microscopy (E-SEM). We first describe how extremely elongated droplets could form on mildly hydrophilic surfaces, naturally contaminated in real environmental conditions. In this configuration, the dew droplet shape can be dynamically and reversibly varied by controlling the out-of-equilibrium conditions associated to condensation/evaporation kinetics. As example of application, we propose a “morphological” sensor that exploits the shape of the dew droplets as transduction mode for detecting organic vapours in the outer atmosphere. Importantly, this study is underlining that environmental stable, purely hydrophilic surfaces can be smartly engineered to induce wetting phenomena at very small scale never observed so far for hydrophobic or heterogeneous surfaces. Our versatile approach based on nanoimprinted, transparent sol-gel films could open great perspectives for the implementation of environmentally stable mildly hydrophilic materials for “ dew engineering” applications such as open-microfluidics, fuming for fingerprints revealing, vapour sensing or water harvesting on glass windows for instance.
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ACS Nano
Wetting is a complex phenomenon that occurs in many situations of everyday life. It is usually described using static and dynamic contact angles, drop-shape anisotropy, and pinning of sessile-drop, in ideal conditions where evaporation or gravitational influences are minimised.1 In real situations, steady-state conditions are not often encountered due to the spatiotemporal fluctuation of many environmental factors, such as relative vapour pressure, convection, or temperature. Dew formation falls into this category since the water droplets form and evolve through condensation/evaporation equilibrium that depends on fluctuations of atmospheric conditions.2 The sequence of events, taking place successively or simultaneously, is rendered even more difficult to assess for real complex surfaces gathering heterogeneity of topography and of surface energy. In the biologic world, Nature has designed efficient surfaces capable of displacing small round droplets to harvest moisture from the atmosphere as in the case of beetle bodies,3, 4 spider silks,5 lizard skins6 or cacti.7, 8 Inspired by biological systems, controlling the formation and the shape of condensed liquids on a surface is of paramount importance in several technological domains including water harvesting,9 anti-fogging optical coatings,10 open-microfluidics11,
12
and fuming fingerprint revealing in crime scene.13 Dew nucleation,
growth, coalescence, self-departure have been studied on artificial model surfaces with various roughness and patterns,2, 14 on hydrophobic,15 heterogeneous hydrophilic/hydrophobic
16-21
, and
slippery surfaces.17 A further step for the development of this rising domain of “dew engineering” consists in fabricating supporting surfaces able to determine the shape of the dew droplets under dynamic, out of equilibrium conditions. In previous reports, a control of the dew droplets shape could be demonstrated on heterogeneous hydrophilic/hydrophobic patterns by pinning the contact lines at predetermined locations.18, 21 However, fabrication of such chemical patterns implies patterning and/or organic functionalization steps that are subjected at damaging or pollution in real environmental conditions. On the other hand, although they are more stable and utilized in many practical applications (such as glass
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windows), purely hydrophilic inorganic surfaces have been less considered as support for dew condensation and shaping. Recently, one report describing the properties of the peristome surface of Nepenthes alata has brought into light the potentiality of hydrophilic surfaces as support for water transport.22 In the present report, we experimentally demonstrate that simple nanopatterned hydrophilic transparent surfaces as support to condensate stable femtoliter (submicron scale) water droplets exhibiting extreme elongation and tuneable shape. Square, elongated, or circular droplets were obtained with square, line, or concentric patterns, respectively and were visualised in situ during formation and recession using optical microscopy and Environmental Scanning Electronic Microscopy (E-SEM). In a specific range of contact angle, we reveal the appearance of Giant Wetting Anisotropy consisting in femtoliter droplets characterized by extreme elongation ratio, never reported so far.23 We will further show that the dew droplet shape can be dynamically varied by controlling the out-of-equilibrium conditions associated to condensation/evaporation kinetics. The patterned surfaces were composed of Anatase TiO2, a material widely used for its interesting physical-chemical properties, extreme stability, natural abundance, and non-toxicity. They were prepared from a sol-gel processing involving liquid deposition, nanoimprinting by Degassing Assisted Pattering (DAP)24 and thermal curing.25 Depending on the pattern geometry, this behaviour was optimal for a specific water contact angle measured on a plain surface, and obtained in natural atmospheric environment.20 Finally, we describe a concept of dew-mediated vapour sensing of volatile organic compounds that exploits the droplet shape as transduction mode. These results reveal the wetting potentials of pure hydrophilic surfaces at small scale, that can be further implemented in domains such as water harvesting,8 sensing,26 “open” digital microfluidics,11 or thermal management.27
RESULTS AND DISCUSSION
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ACS Nano
Dew Shaping Capabilities of Nanoimprinted Hydrophilic Surfaces
Figure 1 shows SEM micrographs of typical TiO2 surfaces used for the present investigations. The nanostructured surfaces, featuring square arrays, or linear and concentric grooves, respectively were prepared by direct soft-nanoimprinting lithography of sol-gel based films as illustrated in S1. The technology allowed patterning of relatively large scale (several cm2) on several substrates. Details on the geometry and dimensions of the patterns in Figure 1 were deduced from SEM and are given in Table in S1. The study of dew formation was carried out using a closed optical microscope equipped with an ultra fast CCD camera, and a chamber equipped with a transparent window for observation (see S2 for details). In this first set of experiments, dew was generated by supersaturating the water vapour pressure in the chamber, just above the surface. To do so, air with fixed PH2O/P0H2O=0.25 was flowed (0.5 L.min-1) at 25°C over the TiO2 surface, while the latter is slowly cooled down from typically 25°C to 5°C (≈10°C.min-1). Droplets start to form above water vapour saturation due to temperature drop, and their growth can be stopped at any time by cutting off the air flux. In this particular experiment, TiO2 surfaces were aged a few days under the ambient atmosphere of the laboratory and were not specifically cleaned before used in order to match real environmental conditions. Water static contact angle investigations performed on plain (non patterned) zones of these same samples gave θ=40° (± 2°) after ambient aging, against θ
