ARTICLE pubs.acs.org/JPCC
Study of Wetting and Adhesion Interactions between Water and Various Polymer and Superhydrophobic Surfaces Benedict Samuel,† Hong Zhao, and Kock-Yee Law* Xerox Corporation, Xerox Research Center Webster, 800 Phillips Road, 147-59B Webster, New York 14580, United States
bS Supporting Information ABSTRACT: The wetting and adhesion characteristics of 20 different surfaces have been studied systematically by both static water contact angle (θ) and dynamic contact angle measurement techniques: sliding angle (R) and advancing (θA) and receding (θR) contact angles. These surfaces cover surfaces of all traits, from smooth and flat to rough and artificially textured. Fourteen of the surfaces are flat, and they range from molded plastic sheets to solution coated polymer films to chemical vapor deposition polymerized polymer films and to self-assembled monolayers on Si wafers. The rest of the surfaces include 4 fluorosilane coated textured Si wafer surfaces and two natural surfaces derived from the front and back side of the rose petal. Static water contact angle data suggest that these surfaces vary from hydrophilic with θ at ∼71° to superhydrophobic with θ exceeding 150°. Plots of θ of these surfaces versus R, (cos θR cos θA), and the contact angle hysteresis (θA θR) all yield scattered plots, indicating that there is little correlation between θ and R, (cos θR cos θA) and (θA θR). Since the later three parameters have been mentioned to relate to adhesion semiempirically between a liquid droplet and the contacting surface, the present work demonstrates with generality that contact angle indeed does not relate to adhesion. This is consistent with a known but not well recognized fact in the literature. In this work, we study both the wetting and adhesion forces between water and these 20 surfaces on a microelectromechanical balance (tensiometer). When the water drop first touches the surface, the attractive force during this wetting step was measured as the “snap-in” force. The adhesion force between the water drop and the surface was measured as the “pull-off” force when the water drop separates (retracts) from the surface. The snap-in force is shown to decrease monotonously as θA decreases and becomes zero when θA is >150°. The very good correlation is not unexpected due to the similarity between the wetting and the “snap-in” process. The analysis of the pull-off force data is slightly more complicated, and we found that the quality of the watersurface separation depends on the surface “adhesion”. For surfaces that show strong adhesion with water, there is always a small drop of water left behind after the water droplet is pulled off from the surface. Despite this complication, we plot the pull-off force versus R, (cos θR cos θA) and (θA θR), and found very little correlation. On the other hand, the pull-off force is found to correlate well to the receding contact angle θR. Specifically, pull-off force decreases monotonically as θR increases, suggesting that θR is a good measure of surface adhesion. Very interestingly, we also observe a qualitative correlation between θR and the quality of the pull-off. The pull-off was found to be clean, free of water residue after pull-off, when θR is >∼90° and vice versa. The implications of this work toward surface contact angle measurements and print surface design are discussed.
’ INTRODUCTION Fundamental understanding of wetting (attractive) and adhesive interactions between imaging materials, such as ink and toner, on various print surfaces is crucial in print engine design and print process development in the printing industry. For instance, in the case of electrophotographic printing, toner image is first developed onto the latent electrostatic image on the photoconductor followed by transfer of the toned image to paper and then fusing to produce a print.1 If the residual toner is “sticking” to the photoconductor surface after toner transfer, a cleaning subsystem will be required to ensure reproducible highquality printing.2 Similarly, in the fusing step,3,4 if the molten toner is adhering strongly on the fuser surface, particularly when the adhesion force between the toner and the fuser surface is r 2011 American Chemical Society
stronger than the cohesive force within the toner patch, the toner image may break and a small amount of toner may leave behind the fuser surface after fusing. This will result in image quality defect, commonly known as offset.5 Similarly in inkjet printing, surface interactions between ink and the printhead6 and the printed substrates79 are known to be extremely important to the printing performance. Contact angle measurement has widely been used to model wetting or dewetting behavior of materials on various surfaces.10 The use of water as the probing liquid is very common, and surfaces with high water contact Received: April 7, 2011 Revised: June 1, 2011 Published: June 27, 2011 14852
dx.doi.org/10.1021/jp2032466 | J. Phys. Chem. C 2011, 115, 14852–14861
The Journal of Physical Chemistry C angles are highly hydrophobic and they are usually lower in surface energy.1113 What is not well recognized is that high water contact angle and low surface energy are not necessary correlating to surface wettability and surface adhesion.1417 For examples, Tsai, Chou, and Penn15 reported the lack of correlation between contact angle data and the adhesive performance for the smooth surfaces of a series of Kevlar fiber. Murase and Fujibayashi16 found that while the surface of their fluorinated polymer exhibited a higher water contact angle (117°) than that of the poly(dimethysiloxane) (PDMS) sample (102°), the interactive energy for the fluoropolymer is 3 times higher (∼50.89 versus 15.64 mJ/m2). Silicones and fluoropolymers, e.g., Teflon (PTFE), are two classes of well-known low surface energy materials that are frequently used in the fusing components in the printing industry.18,19 Both materials are highly hydrophobic with PTFE having a slightly higher water contact angle than PDMS (112117° for PTFE as compared to 102103° for PDMS). However, PTFE was consistently shown to adhere stronger to water and has a larger sliding angle.16,17 To an extreme, Gao and McCarthy20 even called Teflon hydrophilic. The fundamental questions we have to ask are as follows: What does contact angle mean to surface attraction and adhesion? Are these interactive forces in any way correlating to the surface properties? These questions are not new. They had been asked earlier and remain to be clarified.10 Our interests in this area are multifold. We are interested in the fundamental interactive forces when a drop or a patch of imaging material is interacting with a print surface. Of particular concerns are the balance of the adhesive forces at the involving interfaces and the cohesion within the imaging material. How are these interactive forces being measured? Will they correlate to any data generated from the conventional contact angle measurement techniques, such as static contact angle, advancing contact angle, receding contact angle, and sliding angle? In this work, we first study the surface properties of a variety of surfaces with varying hydrophobicity. These surfaces range from smooth and flat to rough and textured, and they vary from hydrophilic to superhydrophobic according to their contact angles. We initially attempted to measure their attractive and adhesive interactions with organic liquids, such as hexadecane and molten solid ink on a microelectromechanical balance system (tensiometer). Our aim is to examine if there is any correlation between the force data and the surface properties. Our initial scoping experiments were not successful due to difficulties in handling low surface tension liquids, such as hexadecane and molten wax ink (2035 mN/m), on the tensiometer. For the sake of experimental procedure development and also to find out if there are correlations between attractive/adhesive forces and the surface properties, we decided to use water (surface tension ∼72.5 mN/m) in our model investigation. Here we21 report the static and dynamic (advancing, receding and sliding) contact angle measurements of water on 20 different surfaces, from smooth and flat to rough and textured. These surfaces vary from hydrophilic to superhydrophobic, and their interactions with water were studied on the tensiometer. Our results reveal with generality that there is little correlation between the static water contact angle and parameters that are commonly thought to be related to adhesion (contact angle hysteresis and sliding angle). Through systematic study, we show that the wetting force measured on the tensiometer correlates well to the advancing contact angle and that the adhesion force correlates well to the receding contact angle.
ARTICLE
’ EXPERIMENTAL SECTION Materials. The polymer surfaces used in this work were obtained from the following sources: poly(tetrafluoroethylene) (PTFE sheet from Dalau Inc., Merrimack, NH), Plexiglas (transparent plastic sheet from Hobby Lobby), and polyimide (5 mil polyimide film from Upilex, Japan). These surfaces were cleaned with 2-propanol before use. The FOTS surface was obtained by self-assembling a perfluoroalkyl chain on a silicon wafer from tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane using the molecular vapor deposition technique on a MVD100 reactor from Applied Microstructures,22 OTS was a selfassembled monolayer prepared from octadecyltrichlorosilane on a Si wafer via a solution technique.23 Thin films of initiated chemical vapor deposition (i-CVD) PTFE, i-CVD silicone, and i-CVD fluorosilicone were purchased from GVD Corp. (Cambridge, MA), and these surfaces were prepared by polymerizing the appropriate precursor monomers on a silicon wafer using the initiated chemical vapor deposition technique.24,25 Films of a perfluoroacrylate polymer and a hydrophobic sol gel coating were acquired externally under a nonanalysis agreement. Polycarbonate was a thin film coated from Makrolon (a high molecular weight polycarbonate from Mobay Chemical Co.). The polyurethane (PU) coating was prepared by mixing a hydroxyl-terminated polyacrylate (Desmophen A870 BA in butyl acetate) with hexamethylene diisocyanate (Desmodur N-3300A) in ketones in the presence a small amount of catalyst (dibutyl tin dilaurate), followed by draw-down coating on Mylar and heat curing at 130 °C for 30 min. On two occasions, a surface modifier, Silclean 3700 from BYK, was used to modify the hydrophobicity of the polymer surface. FOTS modified textured surfaces with regular pillar arrays were prepared on a silicon wafer using photolithography followed by surface fluorosilanation with tridecafluoro-1,1,2,2-tetrahydrooctyltrichlorosilane via the molecular vapor deposition technique.26 The rose petals studied in this work were freshly collected from the garden of one of the authors in Penfield, NY. Instruments and Measurement Procedures. Contact angle was determined using the sessile drop method. The experiments were conducted on an OCA20 goniometer from Dataphysics, which consists of a computer-controlled automatic liquid deposition system and a computer-based image processing system. Details of the measuring method and analysis have been given elsewhere.27 Deionized water (18 MΩ 3 cm, purified by a reverse osmosis process) was used in all measurements. In a typical static contact angle measurement, ∼5 μL of the water droplet was gently deposited on the testing surface using a microsyringe and the static angle was determined by computer software (SCA20) and each reported data is an average of more than five independent measurements. Typical contact angle measurement error is ∼2°. For dynamic measurements, the advancing angle was measured by adding a small volume of water to the surface and the receding angle was measured by slowly removing water from the drop (0.15 μL/s). Tilting angle measurement was done by tilting the base unit at a rate of 1 deg/s with a ∼10 μL droplet using titling base unit TBU90E. All measurements were averaged from five to eight measurements, using a pristine area of the substrate for each measurement. The tilted angle is defined as the angle where the test liquid droplet starts to slide (or move). The interaction of water with different surfaces was studied on a high-sensitivity microelectromechanical balance system or a 14853
dx.doi.org/10.1021/jp2032466 |J. Phys. Chem. C 2011, 115, 14852–14861
The Journal of Physical Chemistry C
ARTICLE
Table 1. Contact Angle Measurement Data and Wetting and Adhesion Force Data of Water on Different Surfaces no.
polymer surfaces
θ, dega
θA, degb
θR, degc
R, degd
cos θR cos θA
snap-in force (μN)
pull-off force (μN)
1
polyurethane (PU)
70.5
85
48.9
51
0.57
471.4 ( 14.2
179.4 ( 2.2
2
PU2% Silclean
98.2
104.3
76.3
31
0.48
316.9 ( 17.7
175.7 ( 3.2
3
PU8% Silclean
104.3
105.9
88.1
23
0.31
292.3 ( 7.3
172.6 ( 2.0
4
polyimide
80.1
82.5
56.1
26.4
0.43
442 ( 54.4
167.3 ( 10.3
5
Plexiglas
86.5
93.9
77.3
29.1
0.29
387.6 ( 13.4
157.4 ( 8.1
6
polycarbonate
92.4
98.2
68.1
59.2
0.52
338.7 ( 22.4
163.2 ( 1.9
7
i-CVD silicone
87.9
91.2
62.2
e
0.49
379 ( 14.2
175.5 ( 2.6
8 9
i-CVD fluorosilicone i-CVD PTFE
115.9 127.7
118 134.9
90.3 73.6
18.2 e
0.46 0.99
141 ( 6.3 72.4 ( 4.2
148.7 ( 1.1 168.8 ( 1.5
10
SAM OTS
109
117.4
94.6
13
0.38
197 ( 9.5
141.3 ( 0.2
11
SAM FOTS
107.3
116
95
13.6
0.35
226.6 ( 8.7
139.5 ( 1.8
12
perfluoroacrylate
113
113.1
61
e
0.88
398.7 ( 18.7
149.2 ( 12.6
13
hydrophobic sol gel
112.2
111.6
92.4
5.6
0.33
197.9 ( 12.2
111.4 ( 8.3
14
PTFE
117.7
126.6
91.9
64.3
0.56
89.7 ( 15.4
162.0 ( 6.6
149 156.2
160 161.3
130.8 142.6
20 10.1
0.29 0.15
0 0
29.4 ( 2.6 15.6 ( 1.7
textured silicon surface (pillar diameter/spacing) 15 16
3 μm/4.5 μm 3 μm/6 μm
17
3 μm/9 μm
154.1
159.9
148.9
5.7
0.08
0
8.54 ( 0.8
18
3 μm/12 μm
156.2
160.8
151.8
3.4
0.06
0
4.71 ( 0.7
19
rose petal, front
144.7
150.7
131.6
20
rose petal, back
132.4
136.6
85.7
6.1 e
0.21
2.9 ( 1.3
31.5 ( 8.0
0.80
23.4 ( 5.7
140.0 ( 15.8
a
Static contact angle, estimated error
