In the Laboratory edited by
Tested Demonstrations
Ed Vitz Kutztown University Kutztown, PA 19530
A Demonstration of Surface Tension and Contact Angle submitted by:
Hyman D. Gesser Department of Chemistry, University of Manitoba, Winnipeg, MB R3T2N2, Canada;
[email protected] checked by:
Paul Krause Department of Chemistry, University of Central Arkansas, Conway, AR 72035
The contact angle of a liquid on a flat surface (1, 2) determines the spreading or wetting properties of the system. This is an important aspect of the flow of liquids as well as the adhesion characteristics of a system. Surfactants can decrease the contact angle, thereby decreasing the surface tension of a liquid. The Figure 1. A drop of liquid is placed on a solid surface. The angle formed contact angle θ of a sessile drop on a surface is shown at the air–liquid–solid interface is called the contact angle. If the liquid– in Figure 1, where the surface is characterized as solid contact area or the volume of liquid is increasing, then the angle meahydrophilic or hydrophobic depending on the value sure is the advancing contact angle, θA . If the liquid–solid contact area or of θ for water on the surface. the volume of liquid is decreasing, then the receding contact angle, θR , is The contact angle normally has two values, dependdetermined. ing on whether the liquid–solid contact area is increasing (advancing contact angle, θA) or decreasing (receding contact angle, θR). The average of these two values is normally called the contact angle, θ. The ratio of these values is related to the surface roughness. Some values of contact angle are given in Table 1. The sessile drop can be projected on an overhead projector. Some examples are shown in Figure 2. A clean glass surface is hydrophilic owing to the presence of the ≡Si–OH groups. Such surfaces can be made hydrophobic by silanization of the surface with trimethylchlorosilane (5% in toluene). The effect is shown in Figure 2, where comparison is made of different liquids on various solid surfaces. The flow of water through hydrophilic microporous membranes was studied by Errede (3). He showed that the decrease in flow rate of filtered tap water as the volume of water that flowed through the membrane increased was not due to the blockage of the pores by microdebris but to the adsorption of organic matter onto the walls of the pores, making them hydrophobic and increasing the contact angle (4, 5) and thereby preventing the water from entering the Figure 2. Projected sessile drops of oil, mercury, and water on microchannels. The Demonstration A small hole (1 mm in diameter or less) is drilled through the center of a thin plastic lens (plastic watch glass or flat sheet). One of the checkers has used a plastic well plate and drilled a 1⁄16-in. hole in each of 6 wells using an untreated well for comparison. A few drops of water are then added to the surface over the hole. If the hole is small enough the water will not flow through it owing to the surface tension of water and the high contact angle of water on the plastic surface. The water is removed and the lens is placed on a metal surface. A Tesla coil is used to spark the hole for a minute or two, though care must be taken to avoid melting the plastic 58
various surfaces: 1, glass; 2, silanized glass; 3, untreated PMMA; 4, hydroxylated PMMA; 5, paraffin-coated glass.
Table 1. Contact Angles for Various Substances θ (°) θR (°) Water on θA (°) Clean glass
0
—
—
Paraffin
108
—
—
Teflon
110
—
—
Graphite
86
—
—
Kel-F
90
—
—
Silanized glass
—
108a
76a
aAt
22 °C.
Journal of Chemical Education • Vol. 77 No. 1 January 2000 • JChemEd.chem.wisc.edu
In the Laboratory
because of the heat generated. Water added to the lens now flows through the hole without hindrance. The sparks from the Tesla coil oxidize and hydroxylate the plastic surface of the hole, converting the hydrophobic surface of the plastic into a wetting hydrophilic surface. The contact angle of water on the plastic is reduced from about 60° to about 20°, allowing the water to flow through the hole. This demonstration can be readily shown to a large class by holding the plastic in the optical path of an overhead projector. Rationale Hydrophilic surfaces in the human body include the cornea and the mouth. Contact lenses, which are hydrophilic, float on a water tear. Prior to the introduction of the soft hydrophilic lens the material in common use was polymethylmethacrylate (PMMA), which has a contact angle with water of about 60°. When PMMA lenses are treated with OH free radicals the surface becomes hydrophilic and the contact lens is more comfortable (6 ). The OH free radicals are readily generated by an electric discharge through water vapor (7) or by Fenton’s reagent (8). The hydroxylation of Teflon
allows ordinary epoxy adhesives to be effective in forming a lap joint of two Teflon strips (9). Similarly, when artificial dentures composed of PMMA are made hydrophilic they adhere to the palate more strongly. This has been done by either depositing a layer of silica or reacting the surface with hydroxyl free radicals (10). Literature Cited 1. Ihrig, J. L.; Lai, D. Y. F. J. Chem. Educ. 1957, 34, 196–198. 2. Young, J. A.; Phillips, R. J. J. Chem. Educ. 1966, 43, 36–37. 3. Errede, L. A.; Martinucci, P. D. Ind. Eng. Chem. Prod. Res. Div. 1980, 19, 573–580. 4. Errede, L. A. J. Membrane Sci. 1984, 20, 45–61. 5. Errede, L. A. J. Colloid Interface Sci. 1984, 100, 414–422. 6. Gesser, H. D.; Warriner, R. E.; Funt, B. L. Am. J. Ophthalmol. Arch. Am. Acad. Ophthalmol. 1965, 42, 321–324. 7. Venugopalan, M.; Jones, R. A. Chemistry of Dissociated Water Vapor and Related Systems; Interscience: New York, 1968. 8. Walling, C. Acc. Chem. Res. 1975, 8, 125–131. 9. Gesser, H. D.; Long, L. J. Polym. Sci. 1967, B5, 469–470. 10. Gesser, H. D.; Castaldi. C. R. J. Prosthetic Dent. 1971, 25, 236–243.
JChemEd.chem.wisc.edu • Vol. 77 No. 1 January 2000 • Journal of Chemical Education
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