This paper is published as part of Faraday Discussions volume 146:

Wetting Dynamics of Hydrophobic and Structured Surfaces

Introductory Lecture Exploring nanoscale hydrophobic hydration

Peter J. Rossky, Faraday Discuss., 2010

DOI: 10.1039/c005270c

Papers

Dynamical superhydrophobicity Mathilde Reyssat, Denis Richard, Christophe Clanet and David Quéré, Faraday Discuss., 2010 DOI: 10.1039/c000410n

Superhydrophobic surfaces by hybrid raspberry-like particles Maria D'Acunzi, Lena Mammen, Maninderjit Singh, Xu Deng, Marcel Roth, Günter K. Auernhammer, Hans-Jürgen Butt and Doris Vollmer, Faraday Discuss., 2010 DOI: 10.1039/b925676h

Microscopic shape and contact angle measurement at a superhydrophobic surface Helmut Rathgen and Frieder Mugele, Faraday Discuss., 2010 DOI: 10.1039/b925956b

Transparent superhydrophobic and highly oleophobic coatings Liangliang Cao and Di Gao, Faraday Discuss., 2010 DOI: 10.1039/c003392h

The influence of molecular-scale roughness on the surface spreading of an aqueous nanodrop Christopher D. Daub, Jihang Wang, Shobhit Kudesia, Dusan Bratko and Alenka Luzar, Faraday Discuss., 2010 DOI: 10.1039/b927061m

Discussion

General discussion Faraday Discuss., 2010 DOI: 10.1039/c005415c

Papers

Contact angle hysteresis: a different view and a trivial recipe for low hysteresis hydrophobic surfaces Joseph W. Krumpfer and Thomas J. McCarthy, Faraday Discuss., 2010 DOI: 10.1039/b925045j

Amplification of electro-osmotic flows by wall slippage: direct measurements on OTS-surfaces Marie-Charlotte Audry, Agnès Piednoir, Pierre Joseph and Elisabeth Charlaix, Faraday Discuss., 2010 DOI: 10.1039/b927158a

Electrowetting and droplet impalement experiments on superhydrophobic multiscale structures F. Lapierre, P. Brunet, Y. Coffinier, V. Thomy, R. Blossey and R. Boukherroub, Faraday Discuss., 2010 DOI: 10.1039/b925544c

Macroscopically flat and smooth superhydrophobic surfaces: Heating induced wetting transitions up to the Leidenfrost temperature Guangming Liu and Vincent S. J. Craig, Faraday Discuss., 2010 DOI: 10.1039/b924965f

Drop dynamics on hydrophobic and superhydrophobic surfaces B. M. Mognetti, H. Kusumaatmaja and J. M. Yeomans, Faraday Discuss., 2010 DOI: 10.1039/b926373j

Dynamic mean field theory of condensation and evaporation processes for fluids in porous materials: Application to partial drying and drying J. R. Edison and P. A. Monson, Faraday Discuss., 2010 DOI: 10.1039/b925672e

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Molecular dynamics simulations of urea–water binary droplets on flat and pillared hydrophobic surfaces Takahiro Koishi, Kenji Yasuoka, Xiao Cheng Zeng and Shigenori Fujikawa, Faraday Discuss., 2010 DOI: 10.1039/b926919c

Discussion

General discussion Faraday Discuss., 2010 DOI: 10.1039/c005416j

Papers

First- and second-order wetting transitions at liquid–vapor interfaces K. Koga, J. O. Indekeu and B. Widom, Faraday Discuss., 2010 DOI: 10.1039/b925671g

Hierarchical surfaces: an in situ investigation into nano and micro scale wettability Alex H. F. Wu, K. L. Cho, Irving I. Liaw, Grainne Moran, Nigel Kirby and Robert N. Lamb, Faraday Discuss., 2010 DOI: 10.1039/b927136h

An experimental study of interactions between droplets and a nonwetting microfluidic capillary Geoff R. Willmott, Chiara Neto and Shaun C. Hendy, Faraday Discuss., 2010 DOI: 10.1039/b925588e

Hydrophobic interactions in model enclosures from small to large length scales: non-additivity in explicit and implicit solvent models Lingle Wang, Richard A. Friesner and B. J. Berne, Faraday Discuss., 2010 DOI: 10.1039/b925521b

Water reorientation, hydrogen-bond dynamics and 2D-IR spectroscopy next to an extended hydrophobic surface Guillaume Stirnemann, Peter J. Rossky, James T. Hynes and Damien Laage, Faraday Discuss., 2010 DOI: 10.1039/b925673c

Discussion

General discussion Faraday Discuss., 2010 DOI: 10.1039/c005417h

Papers

The search for the hydrophobic force law Malte U. Hammer, Travers H. Anderson, Aviel Chaimovich, M. Scott Shell and Jacob Israelachvili, Faraday Discuss., 2010 DOI: 10.1039/b926184b

The effect of counterions on surfactant-hydrophobized surfaces Gilad Silbert, Jacob Klein and Susan Perkin, Faraday Discuss., 2010 DOI: 10.1039/b925569a

Hydrophobic forces in the wetting films of water formed on xanthate-coated gold surfaces Lei Pan and Roe-Hoan Yoon, Faraday Discuss., 2010 DOI: 10.1039/b926937a

Interfacial thermodynamics of confined water near molecularly rough surfaces Jeetain Mittal and Gerhard Hummer, Faraday Discuss., 2010 DOI: 10.1039/b925913a

Mapping hydrophobicity at the nanoscale: Applications to heterogeneous surfaces and proteins Hari Acharya, Srivathsan Vembanur, Sumanth N. Jamadagni and Shekhar Garde, Faraday Discuss., 2010 DOI: 10.1039/b927019a

Discussion

General discussion Faraday Discuss., 2010 DOI: 10.1039/c005418f

Concluding remarks

Concluding remarks for FD 146: Answers and questions Frank H. Stillinger, Faraday Discuss., 2010 DOI: 10.1039/c005398h

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Preface

Alenka Luzar*a and Hugo K. Christenson*b

DOI: 10.1039/c005486k

Hydrophobic surfaces are ubiquitous in nature and in technology, and their impor-tance has long been recognised. More recently, interest has grown in structuredsurfaces as a result of research into naturally occurring hydrophobic surfaces suchas those of many plant leaves (the Lotus effect). It is now possible to engineersurfaces to show a range of properties related to, but not confined to, the traditionalconcepts of hydrophobicity. Non-wetting of a surface may thus be achieved not onlyby minimising the surface free energy, as with a classical hydrophobic surface likeTeflon, but also via an appropriately tailored surface morphology. As a consequence,even low-energy liquids may dewet a surface and the term hydrophobicity is replacedby the more general term ‘‘lyophobicity’’. Theoretical interest in hydrophobic inter-actions and wetting has also been stimulated in several ways. Work on designingsuperhydrophobic surfaces has led to renewed interest in the theories of heteroge-neous wetting due to Wenzel and Cassie–Baxter. Various surface reflectivitymeasurements have been interpreted as evidence for a layer (albeit thinner thanthe diameter of a water molecule) of depleted water density next to extended hydro-phobic surfaces, and different techniques have been adopted to investigate theboundary conditions of flow next to both smooth and structured hydrophobicsurfaces and the relationship to dewetting. Submicroscopic bubbles (‘‘nanobubbles’’)have been discovered on many hydrophobic surfaces in water. Besides giving rise toa long-range attractive force that has been confused with a ‘‘hydrophobic attrac-tion’’, they have raised the question of the importance of dissolved gas for wettingand surface properties in general.

The practical importance of wetting dynamics at hydrophobic and structuredsurfaces is considerable. The list of applications in many industrial and biologicalprocesses includes microfluidics, electrowetting and cell motility. The capillarydriven motion of fluid through structures on a surface bears tremendous importancein the emerging field of nanofluidics and sensor development. The field of electrowet-ting continues to rapidly expand in applications ranging from lab-on-a-chip, liquidlenses and displays, to microelectronics. Surfaces showing significant drag reductionin liquids, as well as a decrease in turbulence at high flow rates may be constructed.This would lead to more efficient movement through liquid, e.g. of propellers, boats,ships and torpedos, etc. Surfaces engineered to be self-cleaning would reduce foulingand contamination, hence leading to longer working life. Rust-resistant surfaces,and anti-fog surfaces may be designed to prevent the growth of discrete dropletscondensing from vapour. The potential applications of such surface engineeringare numerous, and a common theme is greater efficiency in many industrial anddomestic processes. Among more traditional areas of technology hydrophobicityis of key importance in mineral flotation, where efficient bubble attachment oftenrequires surface modification through the use of additives. Most coal and petroleumproducts are hydrophobic and often render surfaces that come into contact withthem hydrophobic. Because of their inherently low surface energy hydrophobicsurfaces present a problem for many applications involving paints, coatings and

aDepartment of Chemistry, Virginia Commonwealth University, Richmond, VA, USA. E-mail:aluzar@vcu.edubSchool of Physics and Astronomy, University of Leeds, Leeds, LS2 9JT, UK. E-mail: h.k.christenson@leeds.ac.uk

This journal is ª The Royal Society of Chemistry 2010 Faraday Discuss., 2010, 146, 9–12 | 9

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adhesives, although this is also exploited in applications such as non-stick (paintresistant) surfaces.

A Faraday Discussion is the ideal forum for making progress towards a betterunderstanding of these phenomena, and the published Discussion volume providesan invaluable reference to the current state of the field for a wide scientific commu-nity of physical chemists, biologists, engineers, materials scientists and nanotechnol-ogists. We decided to act on this idea and the program started to take shape in late2006, after invaluable input from Colin Bain, Pablo Debenedetti, Bob Evans, JohnFinney, Jim Henderson, Mike Klein, Peter Rossky and John Weeks. We wished tofocus on nonpolar surfaces, with particular emphasis on dynamics and tunablewettability, as well as to emphasise the natural extensions to superhydrophobicityand surfaces with chemical and topological heterogeneities. In the late autumn of2007 Colin Bain, Chairman of the Faraday Standing Committee sent us the wonder-ful news – that our meeting was finally on the official schedule for April 2010! We aregrateful to the committee members who gave their time to sort through the largenumber of abstracts we received for contributed papers and took part in the finaldecision on selecting the best and the most appropriate ones for oral presentations.

It was an innovative event. Firstly, it was only the second time a Faraday Discus-sion had been held outside Europe (in North America) – the first one was at theUniversity of Notre Dame, Indiana in 1963 (on radiation chemistry). The venuewas the historic Jefferson Hotel, in Richmond, Virginia, which provided what weare sure, was an unsurpassed setting for a Faraday Discussion meeting. The locationand the time of the year worked together to ensure a very successful meeting at theheight of spring. On reflection, it was a good idea to bring Faraday Discussions backto US soil after 47 years. This way, we could raise the profile of the meeting andattract more delegates from the US. Secondly, it marked the introduction ofa Faraday Discussion Graduate Research Seminar (FD-GRS) that we organizedat Virginia Commonwealth University (VCU) over the preceding weekend. The‘‘Concluding Remarks’’ by Frank Stillinger further describe the scope and successof FD-GRS, which ended with an outing to Maymont Park (see Fig. 1).

Questionnaire responses after the completion of FD146 demonstrated that all thestudents (the response rate was over 90%) felt much better prepared and less

Fig. 1 Students and mentors in Maymont Park after completion of the FD-GRS. (Photo-graph: Alenka Luzar).

10 | Faraday Discuss., 2010, 146, 9–12 This journal is ª The Royal Society of Chemistry 2010

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intimidated to engage in the FD format of critical and public ‘‘dissection’’ of papers.The published Discussion shows that graduate students posed 30% of the questions.The meeting was a great success, and the associated graduate research seminarprovides a model that we hope many other Faraday Discussion meetings will seekto copy.

The subsequent Faraday Discussion meeting was well attended, with close to 130delegates from over 15 countries, with the majority from the US, but quite a fewfrom as far away as New Zealand, Australia and China. The photo of all (almost)the delegates was taken in the lobby of the Jefferson Hotel, on the magnificentGrand Staircase featured in the classic movie ‘‘Gone with the Wind’’ (Fig. 2).

We succeeded in bringing together communities that might not normally interactvery closely, as they deal with length scales from the nanoscopic to the macroscopic.There was a perfect balance between experiment and theory. During the meeting wediscussed recent breakthroughs in state-of-the-art techniques to control the behav-iour of hydrophobic surfaces under specific conditions; e.g. thermal, optical, electri-cal, mechanical, chemical. Further, we discussed new developments to devisetheoretical and simulation approaches to study nano-surfaces, which dominatenanoscale systems and are necessarily highly complex and heterogeneous. Twovery eminent people in the field; Peter Rossky, and Frank Stillinger, delivered theintroductory lecture of the Faraday Discussion Meeting and the concludingremarks, respectively. In between these there were twenty-two contributed andinvited talks, all of which gave rise to interesting and lively debate. Indeed, onmany occasions the discussions had to be cut short due to time constraints. Theposter session was extremely well subscribed with over 70 contributions. ‘‘Flash’’presentations gave the opportunity for all poster presenters to advertise theirwork using a single PowerPoint slide. Presenting research in a condensed way wasa really good learning experience for young scientists. We congratulate all the

Fig. 2 FD146 delegates taken on the historic Grand Staircase of the Jefferson Hotel. (Photo-graph: Alenka Luzar).

This journal is ª The Royal Society of Chemistry 2010 Faraday Discuss., 2010, 146, 9–12 | 11

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student participants who presented a rich variety of excellent work during the postersessions.

Bob Evans, a member of the scientific committee, was standing in for his Bristolcolleague and RSC-member Mike Ashfold who could not attend, and delivered anentertaining after banquet speech in his inimitable style. Bob referred to the historyof previous FD meetings on related topics and gave his account of the two meetingsthat he attended, FD16 and FD20, both held in the freezing cold of an Oxford winter.The loving-cup ceremony passed without incident, except for the usual difficulty in ex-plaining its conduct to inebriated delegates. It became much clearer to them when theBrazilian ensemble ‘‘Quatro na Bossa’’ started playing ‘‘Loving Cup’’ by Rolling Stones.

The feedback that we have received on FD-146 has been very positive. Numerouspersonal comments from delegates speak of how they enjoyed the meeting froma scientific and organisational point of view. Especially the US delegates, formany of whom this was the first Faraday Discussion, are now sold on this format!We expect that this Faraday Discussion and the published 146 volume will havesignificant impact on this exciting field in the years ahead, and we wish to thankall the contributing authors for their effort and support of our endeavour. Delegatesalso expressed their gratitude at being looked after so well and made to feel sowelcome. This southern hospitality extended to a few ‘‘lucky’’ ones who had toprolong their stay at Jefferson because of Eyafj€allaj€okull.

Our organisational achievements are due to many who worked tirelessly before,during and after the meetings. Firstly, we thank the entire staff of the VCU ChemistryDepartment, and especially Rose. Rose’s energy was unsurpassed in overcoming allthe administrative obstacles to deliver fellowships to students, and honoraria tomentors and poster judges. Shirley, from the College of Humanities and Scienceshelped to ensure that the social events ran smoothly. Chemistry graduate studentsstretched their imagination to design the T-shirt logo (Kyler), and brought the profes-sional Brazilian ensemble to the banquet (Fernando). RSC student stewards andmembers of Alenka’s research group helped with the microphones, poster boardsand computers, and at the registration desk. Meredith and Victor from the Schoolof Engineering at VCU were responsible for ensuring that the FD-GRS meetingwas held in a superb lecture hall with top-notch AVI technology and Victor diligentlyvideo-recorded the lectures. Mohammad could be seen tirelessly taking photos andmovies during both meetings and social events. Ken Wynne had the brilliant idea ofasking those with questions to queue up to overcome the shortage of microphonesat FD146. We thank Fred Hawkridge, Sally Hunnicutt, and Scott Gronert for theirwelcoming speech at the reception for FD146, for introducing the young generationto Michael Faraday during the banquet, and for welcoming delegates in MaymontPark. Last, but not least, we are grateful to Morwenna, Anna and Helen of the RSC.

We acknowledge the Chemical Physics Division of the APS and the Division ofPhysical Chemistry of the ACS for their endorsement of FD146. We thank thefollowing organisations for their generous financial support of FD-GRS andFD146: National Science Foundation (CHE-1016888), US Department of Energy(DE-FG02-10ER16152), American Chemical Society (Division of Colloid andSurface Science) and VCU. Without all the support, the inaugural FD-GRS, theoverwhelming student participation in FD146, and all the social activities duringboth meetings, that contributed to a relaxing and engaging student/mentor atmo-sphere, would not have been possible.

The scientific discussion and educational commitment to the course of research inthis field will continue in cyberspace. During the summer months we will bepreparing a dedicated webpage (www.faraday.vcu.edu) that will serve as a continuingopen-access discussion forum on this topic, along with providing the introductorylectures of the mentors, memorable photos, etc. We hope to ensure that the atmo-sphere established at the FD-GRS and carried over to FD146 will last much longer!

Hugo Christenson (Co-Chair)Alenka Luzar (Co-Chair, Editor)

12 | Faraday Discuss., 2010, 146, 9–12 This journal is ª The Royal Society of Chemistry 2010