Under some conditions water droplets can completely wet micro-structured superhydrophobic surfaces. The dynamics of this rapid process is investigated with ultra-high-speed imaging. Depending on the scales of the micro-structure, the wetting fronts propagate smoothly and circularly or – more interestingly – in a stepwise manner for a smaller periodicity of the microstructure. The latter phenomenon leads to a growing square-shaped wetted area: liquid laterally enters a new row on a slow timescale of milliseconds, once it happens the row then fills itself towards the sides in microseconds (“zipping”)

Borkent, Bram M.

Lammertink, Rob G.H.

Lohse, Detlef

Peters, Alisia M.

Pirat, Christophe

Sbragaglia, Mauro

Tsai, Peichun Amy

Wessling, Matthias

English

University of Twente Research Information

XXII ICTAM, 25–29 August 2008, Adelaide, AustraliaTHE ZIPPING WETTING DYNAMICS AT THE BREAKDOWN OF SUPERHYDROPHOBICITYMauro Sbragaglia∗, Christophe Pirat∗, Alisia M. Peters∗∗, Peichun Tsai∗, Bram M. Borkent∗, Rob G. H.Lammertink∗∗, Matthias Wessling∗∗, & Detlef Lohse∗∗Physics of Fluids, Department of Applied Physics, University of Twente, P.O. Box 217, 7500 AE Enschede,The Netherlands∗∗Membrane Technology Group, Department of Chemical Engineeri g, University of Twente, P.O. Box 217,7500 AE Enschede, The NetherlandsSummary Under some conditions water droplets can completely wet micro-structured superhydrophobic surfaces. Thedynamicsofthis rapid process is investigated with ultra-high-speed imaging. Depending on the scales of the micro-structure, thewetting frontspropagate smoothly and circularly or – more interestingly –in a stepwisemanner for a smaller periodicity of the microstructure. Thelatter phenomenon leads to a growingsquare-shapedwetted area: liquid laterally enters a new row on a slow timescale of milliseconds,once it happens the row then fills itself towards the sides in microseconds (“zipping” ).INTRODUCTIONMicro-structured materials can exhibit superhydrophobicbehaviour with effective contact angles of160o and even higher.These large contact angles lead to a prominent low friction mechanism (the so-called Lotus effect), thereby providing awide range of useful and interdisciplinary applications. The Cassie’s law [1] describes such a state of composite wetting(also called “Cassie Baxter” state): an effective contact angle for the droplet is determined by surface heterogeneities, i.e.surface posts alternating with air pockets trapped below the droplet (Fig. 1a-b). In some cases, this superhydrophobicitycan break down [2]. After some initial infiltration the fluid can spread and the droplet impregnates through the micro-structure, yielding the smaller contact angle of the so-called “Wenzel state” [1] (Fig. 1 d-e, c). Despite good understandingand extensive applications of a pure Cassie-Baxter or a pureWenzel state, the detailed dynamics and physical mechanismof Cassie-Baxter to Wenzel transition are challenging research topics and still remain puzzling [2, 3]. In this paper, thedetails of the transition are studied from the experimental, numerical, and theoretical point of view. Micro-structuredsurfaces with regular arrays of posts are used and allow a quantitative understanding of the transition in terms of thegeometry [4] and liquid-solid interactions [5].Figure 1. Our experimental observation of the fast transition from a Cassie-Baxter to a Wezel state. A drop deposited on a micro-patterned surface (shown in (a)) can stay suspended with airpockets trapped in the grooves underneath the liquid, reveal d in (b).At some point the Cassie-Baxter state spontaneously breaksdown, resulting in a lower contact angle shown in (c). The drop thenhomogeneously wets the substrate—indicated by the dark areas in (d) from a bottom view. The bar in (d) marks a length scaleof100µm. (d)-(f) are the snapshots of the fast dynamics of this transitio in a stepwise manner in milliseconds. The detailedzippingprocess of (f) is enlarged in (g), filling in microseconds.RESULTS AND DISCUSSIONExperimentsWe measured the filling dynamics during the breakdown of the Cassie-Baxter state with a tiny droplet of Milli-Q waterdeposited on micro-structured hydrophobic surfaces. The substrates consist of square pillars arranged on a regular squ elattice with heighth = 10µm, width w = 5µm, and an inter-spacinga between the pillars varying from2 to 26 µm(see Fig.1a). These micro-patterns are precisely fabricated through a micro-molding technique. The substrates used arethin translucent polymer films, including a block-copolymer,“kraton” (styrene-butadiene-styrene) and a rubbery polmer,PDMS (polydimethylsiloxane). The observations are performed from the bottom of the film via an inverted opticalmicroscopy (see Fig.2a). When water impregnates between pillars, the image color turns to filled black. A high speedcamera was used, mostly with a sampling rate of10 000 fps, to capture the rapid filling dynamics of the transition.Theory and PhenomenologyAfter a liquid infiltration between the pillars, an energy argument can be established to introduce a critical length-scaleratio of the gap/height value,(ah)c, above which the filling takes place. The theory is based on a bal nce of the energygained by the reduction of the liquid-gas interface on the top of the pillars and the energy lost in the wetting of themicrostructure, finally resulting in(ah)c=2cos θ + 1− 2with θ the wetting angle on the flat surface of the same material. When t control parametera/h approaches this criticalvalue(ah)c the fluid front propagating from the Cassie-Baxter to the Wenzel state experiences a slowing down: the mainfront is almost stopped in between the pillars. In this regime the filling mechanism is observed to be a zipping that liquidfills the structured material perpendicular to the direction of the main front, revealed in Fig.1 g. This zipping-wetting frontreflects the geometrical arrangement of the pillars, as shown in Fig.1 d-f. In the other limit, whena/h is much larger thanthe critical value, a smoother propagation is observed: thefilling front is faster and is able to fill the micro-structurewiththe development of a circular wetting front (see Fig.2) [2].Figure 2. (color) Bottom views of the front evolution of the transition, as sketched in (a). In (b) three snapshots for the case witha = 5µm are shown, leading to square-shaped wetted area. In (c) a circular wetted area fora = 11µm. Figures (d) and (e) show thecorresponding simulation results by the Lattice Boltzmannmethod witha = 5µm and11µm, respectively.Numerical SimulationsWe performed numerical simulations with a three-dimensional (3D) lattice Boltzmann algorithm for single componentmultiphase fluids to understand the fast transition observed experimentally [6]. Geometrical structures and wetting prop-erties are parametrized according to the experimental values. Consistent with the experimental observations, simulationresults show spherical wetted areas in the largea/h case whereas square-shaped wetted areas for smaller valuesof a/h(Fig.2). The pathway of the single zipping filling shown in Fig. 1 g is also reproduced in these simulations [2]. In usingthe mesoscale algorithm, one has no need to track the moving interfaces when simulating multiphase flows. Thus, thesenumerical simulations offer insight into the zipping-wetting dynamics, which is a 3D complex problem involving multipletimescales [2].CONCLUSIONSWe have experimentally, numerically, and theoretically revealed the origin of zipping wetting behavior at the breakdownof superhydrophobicity, namely, the existence of two different timescales. We observed that the wetting process startlocally from a single point and then proceeds laterally. Close to a critical point the front propagation slows down dueto viscous effects, and zipping wetting is observed. As a consequence, the front propagates in a stepwise manner andsquare-shaped wetted areas emerge. Ongoing extension of this work includes the investigation of scaling relations forthe wetting front velocity and the effects of geometry (i.e., a, h, w, pillar shapes, and micro-patterns) and the wettingproperties.References[1] A. B. D. Cassie and S. Baxter,S. Trans. Faraday Soc.40, 546-551 (1944); R. N. Wenzel,Ind. Eng. Chem.28, 988-994 (1936).[2] M. Sbragaglia, Alisia M. Peters, Christophe Pirat, BramM. Borkent, Rob G. H. Lammertink, Matthias Wessling, and Detlef Lohse.: SpontaneousBreakdown of Superhydrophobicity.Phys. Rev. Lett.99:156001 (2007); C. Pirat, M. Sbragaglia, A. Peters, B. Borkent, R. Lammertink, M.Wessling and D. Lohse, submitted (2008).[3] S. Moulinet and D. Bartolo.: Life and Death of a Fakir Droplet: Impalement transitions on superhydrophobic surfaces. Euro. Phys. E24:251–260(2007).[4] T. Cubaud and M. Fermigier,Europhys. Lett.55, 239-245 (2001).[5] Courbin et al.,Nature Materials6,661-664 (2007).[6] R. Benzi, L. Biferale, M. Sbragaglia, S. Succi, and F. Toschi, Phys. Rev. E74, 021509 (2006).

The Zipping-wetting Dynamics at the Breakdown of Superhydrophobicity

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The Zipping-wetting Dynamics at the Breakdown of Superhydrophobicity

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