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14100 DOI: 10.1021/la903043a Published on Web 09/22/2009 Langmuir 2009, 25(24), 14100–
pubs.acs.org/Langmuir © 2009 American Chemical Society
Wetting and Superhydrophobicity
Lichao Gao and Thomas J. McCarthy*
Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003
Xi Zhang*
Department of Chemistry, Tsinghua University, Beijing 100084, China
Received August 15, 2009. Revised Manuscript Received August 27, 2009
This article is written to serve as an introduction to the topic of “Wetting and Superhydrophobicity” and to give some perspective for the papers that follow. This area of research that impacts and overlaps many fields of science and technology has been identifiable for centuries, but has gained considerable traction in the past decade.
The field of science known as “wetting” (limiting it to the wetting of solids by liquids) is central to innumerable natural processes as well as many human-derived (invented, caused, manufactured) technologies. These could be put into an endless list that would include life-essential, useful, beautiful, exciting, and even dangerous items. This list could include the growing number of studies of solid life forms (examples are beautiful insect wings and plant leaves) that evolved on earth both requiring and needing protection from the earth’s liquid (water). It could also include intricate fluidics devices, efficient condensers, photoresists on silicon, house paint, nonfogging bathroom mirrors, biomi- metic models of various body parts, lethal solutions that can penetrate the protective layers of pests, and treated surfaces of heat-seeking missile optics. This field overlaps, intersects or impacts in some way, most every field of science and every technology. Over the past decade, the field of wetting has gained renewed interest in large part due to what has become known as super- hydrophobicity. We leave the definition of this term for discussion by others; however, extreme water repellency is a sufficient description for these introductory remarks. Evidence for this renewed interest is shown in Figure 1, which plots annual citations to the two seminal publications on extreme hydrophobicity: the 1936 paper^1 of Wenzel that explains how roughness can con- tribute to hydrophobicity, and the 1944 paper^2 of Cassie and Baxter that shows that porous surfaces (functioning as surface mixtures of solid and air) can impart superhydrophobicity. The impressive increase in citation rates of these two papers over the past 10 years is due to numerous key reports during this period that helped accelerate interest, but two early reports stand out as being particularly influential. The 1996 Langmuir paper^3 by Onda et al. (this issue of the journal featured a cover showing a photo- graph of a surface supporting a near-180° sessile water drop) and the 1997 Planta paper^4 by Barthlott and Neinhuis that coined the
term “lotus effect” should be acknowledged for activating much of the interest in wetting demonstrated by Figure 1. Studies on what now would be called superhydrophobicity were carried out prior to the mid-1990s and, indeed, prior to the Wenzel and Cassie & Baxter reports, but, relative to now and the recent years indicated in Figure 1, this area of research received only limited attention.5, To introduce the papers on wetting and superhydrophobicity that follow this article and as well to contribute to the perspective that this group of papers gives to this field of research in 2009, we have one goal in particular: to refer the reader to several perspec- tives of this field that others have had at certain points in time over the past ∼200 years and trace a line of history that begins in 1804 and is ongoing today. The particular points in time and the papers we reference here were chosen to lead the post-2009 reader to these sources for appreciation of additional and different perspectives.
- Quoting Good,^7 “most surface and colloid chemists think of Thomas Young as the father of scientific research on contact angles and wetting.” Good briefly reviews^7 earlier contributions to wetting mentioning Aristotle, Archimedes, and Galileo. Young was a genius and polymath who lived at the turn of the 18th to the 19th century.^8 He was a physician who made major contributions to vision, physiology, sound, light, language, solid mechanics, and Egyptology. He also dabbled in many things, and, one of these, the cohesion of fluids, led to research and analysis that was presented in an essay read to the Royal Society in 1804 and published in 1805.^9 In the second paragraph of his essay, Young makes the following comment:
“But it is necessary to premise one observation, which appears to be new, and which is equally consistent with theory and experiment; that is, that for each combination of a solid and a fluid, there is an appropriate angle of contact between the surfaces of the fluid, exposed to the air, and to the solid.”
† (^) Part of the “Langmuir 25th Year: Wetting and superhydrophobicity”
special issue. *Corresponding author. E-mail: [email protected] (T.J.M.); [email protected] (X.Z). (1) Wenzel, R. N. Ind. Eng. Chem 1936 , 28 , 988. (2) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944 , 40 , 546. (3) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996 , 12 , 2125. (4) Barthlott, W.; Neinhuis, C. Planta 1997 , 202 , 1.
(5) Zhang, X.; Shi, F.; Niu, J.; Wang, Z. J. Mater. Chem. 2008 , 18 , 621. (6) Chen, W.; Fadeev, A. Y.; Hsieh, M. C.; Oner, D.;€ Youngblood, J.; McCarthy, T. J. Langmuir 1999 , 15 , 3395. (7) This quotation is taken from Good, R. J. J. Adhes. Sci. Technol. 1992 , 6 ,
(8) Robinson, A. The Last Man Who New Everything; Penguin: New York, 2006. (9) Young, T. Philos. Trans. R. Soc. London 1805 , 95 , 65. This essay has been digitized and is available at www.google.com/books.
Langmuir 2009, 25(24), 14100–14104 DOI: 10.1021/la903043a 14101
Gao et al. Perspective
This was the first description of what we now refer to as contact angle. He goes on in his essay to examine this observation in incredible (even for today) detail and makes his genius obvious. Young did not know about atomic or molecular structure, chemical bonding, dipoles, molecular interactions, or thermo- dynamics. Gibbs, Helmholtz, and van der Waals were not yet born. Young viewed a liquid as a collection of particles that attract one another giving rise to a “uniform tension of the surface.” What is now understood as surface tension was known to Young and he attributes it to Segner (1751)^10 in his essay and makes the point, “Since the time of Segner, little has been done in investigating accurately and in detail the various consequences of the principle.” Young’s perspective involving particles with short- range attractive forces is shown in Figure 2. The nature of the cohesive forces between particles (molecules) would not be known until the next century, but Young understood that the “cohesion of superficial particles” at the surfaces of the liquid (Figure 2a) and solid (Figure 2b), and the common surface of the solid and liquid (Figure 2c) give rise to forces that balance to give an “appropriate angle of contact” between the liquid and solid. Young did not write an equation, but states clearly, using the word “force”, what others have expressed in equation form (eq 1) as the balance of forces due to the cohesion of superficial particles
FSV ¼ FLV cos θ þ FSL ð 1 Þ
at the three interfaces (FSV, FLV, and FSL). Young also made the following statement in the second-to-last paragraph of his essay:
“it may be assumed as consonant both to theory and to observa- tion, that the contractile force of the common surface of two substances, is proportional, other things being equal, to the differ- ence of their densities.”
Equating surface forces with density shows the depth of Young’s instinct. He did not know of nuclei, protons, neutrons, or atomic weight and of course he did not know about low surface tension perfluoroalkanes, silicones, or polar interactions between liquids and solids. He assumed that dense substances must have very strong attractive forces between their particles. This simple and naive (three-dimensional (3D) density) perspective of Young is worth considering today and leads to obvious suggestions that
are worthy of consideration for experiment: When contacting air, a water drop makes a contact angle of 180°. Young’s perspective would lead to the belief that a solid with a density the same as that of air would also exhibit a contact angle of 180°. With the right surface chemistry, this is likely very close to the truth. This suggests that hydrophobic aerogels (that can have densities of a few milligrams per cubic centimeter) should be superhydro- phobic (air has a density of ∼1 mg/cm^3 ). If Young’s perspective is focused to a surface region, we can make statements such as “Water repellency is inversely proportional to the density of the outermost nanometer of a solid.” This focused Young’s perspective may be more useful to some than modern theories (it is certainly simpler). Young has often been criticized^8 for his lack of detailed experimentation, and it is often unclear from his writing whether or not an experiment had actually been performed. We note that he does not mention anything concerning reproducibility, error, or method of measurement of “an appropriate angle of contact ” for any solid/liquid pair, and he certainly did not observe that static contact angles are irreproducible while advancing and receding contact angles can be measured precisely and reprodu- cibly. His tacit assumption that there is no contact angle hysteresis suggests that he did not make many measurements. Independent of any experiments that he may or may not have carried out, however, his insight into wetting is clear and still applicable after more than 200 years. The qualifications that he makes in his writing (that “cover” his claims - even to readers two centuries later) are worthy of note and we have given two examples in the indented statements above: In the first he says with regard to an appropriate angle of contact, “equally consistent with theory and experiment.” He does not state that it is consistent with either or that no theory or experiment had yet been reported. In the second he says with regard to density differences, “other things being equal,” when he could not have known what these other things were. It is important to realize that the perspective of wetting that
Figure 1. Citations versus year for the Wenzel (black) and Cassie and Baxter (gray) publications (refs 1 and 2). Data taken from ISI Web of Science.
Figure 2. Young’s “particle view” of a liquid (a) and a solid (b) and the liquid-solid interface (c).
(10) Rowlinson, J. In Fundamentals of Inhomogeneous Fluids; Henderson, D., Ed.; Marcel Dekker: New York, 1992; p 1.
Langmuir 2009, 25(24), 14100–14104 DOI: 10.1021/la903043a 14103
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superhydrophobicity requires reading a few more of Bartell’s papers. He published 94 papers from 1909 to 1957, and the majority of them would have been appropriate for Langmuir. Eighty-seven of his papers are in American Chemical Society journals, and we note that eight of them appeared in The Journal of Physical and Colloid Chemistry during the 4 years (1947-1950) that this was the name for The Journal of Physical Chemistry. To show that his insight into wetting was profound and that he had this insight long before 1953 and indeed before Wenzel’s publication, we quote from his 1932 paper, 24 which discusses work he published in 1927: 25
“The existence of advancing and of receding angles was well known, but it was assumed, at least by us, that either an advancing or a receding angle would, within a short time, so adjust itself as to give finally a definite equilibrium angle which would be the same whether approached from the advancing or the receding angle. We have since obtained good evidence that advancing angles and receding angles may each exist as definite, but different, equilibrium angles.”
The three 1953 papers^21 -^23 are titled “Surface Roughness as Related to Hysteresis of Contact Angle” (Parts I, II and III). These papers are well written, straightforward accounts of careful contact angle measurements made on paraffin surfaces that were machined to have pyramid-shaped asperities with different sizes, separations, and angles of inclination. The contact angle and hysteresis values measured are discussed using simple geometric analysis of contact line motion. Bartell saw no need to use Cassie’s analysis, no need to invoke thermodynamics or equilibrium, and disproved Wenzel’s theory by measuring contact angles of a puddle that was over a patch of pyramids. He states:
“The results of this experiment indicate that the contact angle and resultant drop shape are determined at the solid-liquid-air interface and that increasing the surface area beneath the drop by roughening does not alter the contact angle. One must conclude, therefore, that Wenzel’s modification of the Young Equation is not justified for roughness of a microscopic or macroscopic magnitude.”
Two of the surfaces that Bartell prepared (with 60° sloped pyramids) exhibited water contact angles of θA/θR = 158°/125° and θA/θR = 163°/122° and were described as follws:^21 “On these surfaces water drops were exceptionally mobile and rolled freely over the surface on slight tilting.” Figure 4 is from the first of these three papers. The caption is identical to the one used in 1953; φ refers to the angle of inclination of the pyramid features that they studied. 1963 - 1968. This period of time was chosen because four monographs are available that are based on international sym- posia held during this timespan. The Kendall Award Symposium, sponsored by the Division of Colloid and Surface Chemisty and honoring W. A. Zisman, was held at the Los Angeles National ACS Meeting in April, 1963, and a monograph, Contact Angle, Wettability and Adhesion^26 was published. The Division of Industrial and Engineering Chemistry held symposia at National ACS meetings in Washington, DC, in June 1964 and June
- Two monographs, Chemistry and Physics of Interfaces^27 and Chemistry and Physics of Interfaces II,^28 were published. A meeting of the Society of Chemical Industry at the University of
Bristol in September 1966 led to the monograph Wetting.^29 These books show that the perspective of the field of wetting at that time was very mathematical and that contact angle measurements were primarily being used to derive thermodynamic information about solids. A single, “surface energy” parameter was desired. Zisman developed an empirical wetting parameter, critical surface energy (γC), and Good and Fowkes proposed that interfacial tension could be broken into components such as polar and dispersive or dispersive and acid-base. There are multiple papers by these authors in these monographs. In a paper by Adamson and Ling in ref 26 titled “The Status of Contact Angle as a Thermodynamic Property”, an interesting, important, and most often ignored conclusion was made: “In the usual circumstance of solids not near their melting point, these interfaces will not be equilibrium ones, so it appears operationally meaningless even to apply the term of surface free energy to them.” During this period there was little interest in superhydropho- bicity, with the exception of a five-part series of papers by Johnson and Dettre. Parts I and II are in ref 26, Part V is in ref 29, and Parts III and IV were published as journal articles.30,31^ They performed calculations on surfaces with idealized roughness and showed that energy barriers between metastable drop states determine the magnitude of contact angle hysteresis. They also reported experi- mental data on both randomly rough surfaces and patterned surfaces. Figure 5 shows a figure reproduced from Part V of the series of papers. Fluoropolymer surfaces of the structures indi- cated as E and F in the figure exhibited advancing and receding contact angles over 160°. It is clear from this figure that Johnson and Dettre, in 1965, were wrestling with issues of superhydro- phobicity that are being readdressed after more than 40 years.
- This date is included because another monograph^32 was published based on a Symposium honoring R. J. Good,
Figure 4. On a fibrillar surface, φ would have a value of 90°, and, if the stable contact angle were 90° or greater, one would observe an apparent contact angle of approximately 180°. As a result, the water would not penetrate beneath the exterior surface, and air would be entrapped beneath the drop. Reprinted with permission from ref 21.
(24) Bartell, F. E.; Whitney, C. E. J. Phys. Chem. 1932 , 36 , 3115. (25) Bartell, F. E.; Ostrhof, H. J. Ind. Eng. Chem. 1927 , 19 , 1227. (26) Fowkes, F. M., Ed. Contact Angles, Wettability and Adhesion; American Chemical Society: Washington, DC, 1964. (27) Gushee, D. E., Ed. Chemistry and Physics of Interfaces; American Chemical Society: Washington, DC, 1965. (28) Gushee, D. E., Ed. Chemistry and Physics of Interfaces II; American Chemical Society: WA, 1971.
(29) Wetting, S. C. I. Monograph No. 25; Society of Chemical Industry: London,
(30) Johnson, R. E.; Dettre, R. H. J. Phys. Chem. 1964 , 68 , 1744. (31) Dettre, R. H.; Johnson, R. E. J. Phys. Chem. 1965 , 69 , 1507. (32) Mittal, K. L., Ed. Contact Angles, Wettability and Adhesion; VSP: Utrecht, The Netherlands, 1993.
14104 DOI: 10.1021/la903043a Langmuir 2009, 25(24), 14100–
Perspective Gao et al.
sponsored by the Division of Colloid and Surface Chemistry, at the National ACS meeting in San Francisco in April, 1992. This symposium and monograph have the same title, Contact Angle, Wettability and Adhesion, that was used in 1963 to honor W. A. Zisman. There is much to contrast between 1963 and 1992 and between these two volumes with the same name. We make only
three related points: (1) During these 29 years, X-ray photoelec- tron spectroscopy became the quantitative technique to assess surfaces and contact angle measurements were down-played. (2) Good, in the first paper^7 of this symposium (and on page 10 of the monograph), urges investigators of contact angles to measure both advancing and receding angles. He says with regard to a sessile drop measurement, “Obviously, such an angle will be of a lower degree of scientific usefulness than will a true θA or θR.” There are numerous examples in this monograph of where receding contact angles were not measured and where data were just identified as “contact angles.” (3) During the 1980s, self- assembled monolayers (SAMs), very similar to those prepared by Zisman in the 1950s, were developed by Sagiv,^33 Nuzzo,^34 and Whitesides,^35 and these developments have had a profound impact on surface chemistry and indeed the content of Langmuir. Zisman developed methods for careful advancing and receding contact angle analysis of SAMs because this was the only technique he had at his disposal. The 1980s groups had more sophisticated analytical methods and did not choose to measure receding contact angles. 1996 - 2009. There are recent reviews5,36-^40 on wetting and superhydrophobicity that indicate that this field is alive, healthy, and growing. We have chosen to outline historical perspectives to be tacitly self-critical of our “field” that has developed since refs 3 and 4 catalyzed renewed interest in wetting. We note that neither ref 3 nor ref 4 report advancing or receding contact angles. Our critical tone should not be confused with a lack of optimism. The growth indicated by Figure 1 has been and is exciting, even if the theories in these papers cited are fundamentally flawed. The field has grown so fast that it will take time for it to catch up with all the things that happened before it began, but it will. Even the terminology has moved too fast to keep up with. Reference 37 states “At this stage there are many reports using whatever terminology comes to mind, making it unclear exactly what kind of surface is being dealt with,” and “There are therefore many categories of surface where roughness enhances contact angle and not enough official names to define them.” This certainly sounds like an exciting and dynamic area of research, and it is. The papers that follow this one are only “snapshots” of a broad, interdisciplinary, and worldwide field that spans fundamental theoretical and experimental physics (see the papers of McCarthy, Neumann, Amirfazli, Erbil, and Bhushan), many disciplines of chemistry (see the papers of Theato and Matar), biology (botany and zoology; see the papers of Koch and Shirtcliffe), and materials science (see the papers of Jiang, Chen, Han, and Nakajima).
Acknowledgment. L.G. and T.J.M. thank the NSF-sponsored Center for Hierarchical Manufacturing (CMMI-0531171) and Materials Research Science and Engineering Center (DMR-
- at the University of Massachusetts. X.Z. thanks the National Basic Research Program (2007CB808000) for financial Figure 5. Surface configurations for several porous surfaces. support. Reprinted with permission from ref 29. (33) Sagiv, J. J. Am. Chem. Soc. 1980 , 102 , 92. (34) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983 , 103 , 4481. (35) Wasserman, S. R.; Yao, Y.-T.; Whitesides, G. M. Langmuir 1989 , 5 , 1074. (36) Feng, X.; Jiang, L. Adv. Mater. 2006 , 18 , 3063. (37) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008 , 4 , 224. (38) Genzer, J.; Efimenko, K. Biofouling 2006 , 22 , 339. (39) Quere, D. Ann. Rev. Mater. Res. 2008 , 38 , 71. (40) deGennes, P.-G.; Brochard-Wyart, F.; Quere, D. Capillarity and Wetting Phenomena; Springer: New York, 2004.
