Standards for Contact Angle Measurement

Several standards have been written to define methods for measuring contact angle for different applications. The following sections review the definition of contact angle and surface tension, and review the practical significance of these terms. Summaries of four ASTM methods are provided, full text can be purchased through the ASTM web site (www.astm.org) or are available in your library.

Contact Angle
Surface Tension
Practical Significance of Contact Angle and Surface Energy

ASTM D724-99 Standard Test Method for Surface Wettability of Paper (Angle-of-Contact Method)
ASTM D5725-99 Standard Test Method for Surface Wettability and Absorbency of Sheeted Materials Using an Automated Contact Angle Tester
ASTM C813-90(1994)e1 Standard Test Method for Hydrophobic Contamination on Glass by Contact Angle Measurement
ASTM D5946-96 Standard Test Method for Corona-Treated Polymer Films Using Water Contact Angle Measurements
TAPPI T458 Surface Wettability of Paper (Angle of Contact Method) (TAPPI web site: www.tappi.org)

Contact Angle

The phenomenon of wetting or non-wetting of a solid by a liquid is better understood by studying what is known as contact angle. Contact angle methods have been developed extensively over the past four decades. A large body of reliable data has been accumulated and vast literature exists correlating contact angle data with surface properties of tension.

Consider the drop of a liquid resting on a solid surface. The drop of liquid forming an Angle may be considered as resting in equilibrium by balancing the three forces involved. Namely, the interfacial tensions between solid and liquid SL, that between solid and vapor SV and that between liquid and vapor LV. The angle within the liquid phase is known as the contact angle or wetting angle. It is the angle included between the tangent plane to the surface of the liquid and the tangent plane to the surface of the solid, at any point along their line of contact. The surface tension of the solid will favor spreading of the liquid, but this is opposed by the solid-liquid interfacial tension and the vector of the surface tension of the liquid in the plane of the solid surface.

Surface Tension

The most important consideration for our purposes is that the Critical Surface Tension values are intimately related to the surface constitution. Even small changes in the outermost atomic layer are reflected in a change of Critical Surface Tension, while other properties might remain essentially unchanged. For example, a simple hydrocarbon surface, like that of polyethylene, exhibits contact angles leading to a Critical Surface Tension of about 31 dynes/cm. Gradual replacement of hydrogen atoms in the surface by fluorine atoms gradually decreases the critical values to 19 dynes/cm as observed with polytetrafluorothylene. Conversely, gradual replacement of surface hydrogen with chlorine atoms leads to an increased Critical Surface Tension gradually approaching 41 dynes/cm as with polyvinylchlorides.

Practical Significance of Contact Angle and Surface Energy

Studies and measurements of contact angles of liquids on solids have great technological importance. This is especially true with water; every action of water on earth is controlled by its wetting behavior with the solid with which it comes into contact. For example, contact angle of water on our skin is about 90 degrees. If it was zero, water could have penetrated the pores of the skin and possibly been absorbed by blood. The structure of a bird’s feather is constructed such that the contact angle of water on a typical feather is as high as 150 degrees. Machines such as Lathes are usually coated with a thin layer of grease or oil especially during transportation to prevent corrosion by water. This is achieved by the fact that grease or oil layer forms a highly non-wetting system with water. In the manufacture of printing inks, the contact angle formed by a drop of ink on paper determines the printing quality of ink. It has been practically observed that q must be ideally between 90 degrees and 110 degrees. If it’s less than 90 degrees, the ink will spread on paper. If it’s more than 110 degrees, breaks will occur while printing.

The importance of contact angle measurement in the textile and fiber industry need not be overstressed. Cotton yarn is usually wetted by water, but synthetic fabrics have definite contact angle for water. Nylon, for example, gives a contact angle of about 40 degrees. Fabrics must be coated, therefore, with suitable wetting agents. Otherwise, it will be difficult to remove dirt and soil while washing the fabric with water. But it is advantageous to use a non-wetting surface for a raincoat or umbrella cloth. Coating by a silicon polymer gives a highly non-wetting system. Waterproofing or water-repellency is an important industrial process, which depends on contact angle values.

Most of the polymers like polyethylene, polypropylene, Teflon, etc. show high contact angle behavior with many liquids. A number of applications of polymers have taken advantage of this situation. An example is the Teflon coated frying pan. Contact angle measurement has shown that the contact angle is about 35 degrees for cooking oil on Teflon; thus oil must not stick to the surface making it easier for cleaning.

The efficiency of insecticide sprays also depends on their wetting behavior on the surface of insects. Usually in most insecticides, an organic liquid having a low surface tension is used a spray so that it spreads completely. Contact angle is therefore an essential parameter to be considered in any pesticide or insecticide spray formulation.

An important technological application that emerged out of contact angle studies is in the enhanced oil recovery from sand beds. Laboratory experiments on displacing petroleum by water in glass capillaries have demonstrated that a considerable fraction of the oil remains attached to the wall when the central space of the capillary is already filled with water. In a sand column, the amount of oil remaining in the sand when water appeared at the downstream end of the column was greater, greater the contact angle. Flooding the oil wells with surfactants along with water or steam reduces the pressure drop across each oil-water meniscus, reduces the oil-water interfacial tension and changes the contact angle so that water displaces oil at the liquid-solid interface. The process is called ‘tertiary oil recovery’ and it is now possible by this method to recover more than 90% of oil from an oil well.

Understanding of contact angle is also important in the mining industry. An example is froth flotation, the process of concentrating minerals of heavy metals (e.g., zinc blende, galena, chalcopyrite, etc.). This is achieved by agitating finely divided minerals in froth of water and air, so that some float and others sink. The useful minerals become attached to the air bubbles, rise with them into the froth layer and are collected. If the contact angle between the particle-water-air interface is smaller, the particle does not float easily. Therefore this contact angle is increased to as much as 60 degrees by adding ‘collectors’ like alkyl xanthanates.

In heat exchangers and condensers used in chemical industries, maximum efficiency is achieved if the metal surface is coated by a non-wetting agent like calcium stearate or oleic acid. Liquids condensed on such coated walls form droplets because of high contact angle and fall down easily. This technique is called 'dropwise’ condensation. Contact angle measurement is important in controlling droplet surface area to increase or decrease evaporation and to optimize wetting of heat exchanger surfaces.

Another area where contact angle plays an important role is in detergency, the process of cleaning clothes, etc., by a surface-active agent. If the soiled cloth is to get rid of dirt, grease, oil, etc., it is necessary that water must spread and penetrate the dirt particles. If spreading is to occur, the interfacial tension between solid-liquid and liquid vapor must be a s small as possible. What the detergent does is exactly to lower these interfacial tensions by the process of adsorption. Contact angel measurements of lotions, oils, soaps and other toilet preparations on human skin and hair strands provide valuable information for their correct formulation and are now routine.

Knowledge of contact angle behavior of liquid metals on metal and oxide surfaces is essential in understanding the process of soldering, brazing, tinning and heat transfer. Other applications of contact angle measurements are in the field of adhesives, lubricants, surface treatments, polymers, biomedicine, etc. Many more specific examples can be cited, but from those discussed already, it is clear that contact angle plays a significant role in a number of manufacturing industries. For further reading:

References:
Physical Chemistry of Surfaces, 3rd Edn, Arthur W. Adamson, Wiley-Interscience, New York (1976)
Surface Chemistry: Theory and Industrial Applications, Lloyd I. Osipow, Reinhold Publishing Corp., NY (1962)

For more information on surfactants, please visit the Surfactants Virtual Library at www.surfactants.net

D724-99 Standard Test Method for Surface Wett-ability of Paper (Angle-of-Contact Method)

1. Scope

1.1 This test method covers the quantitative determination of the resistance of paper surfaces to wetting by measuring the behavior of a drop of liquid when applied directly to the paper surface.

1.2 The procedure described in this test method involves the measurement of the angle of contact. Both the initial wettability and the rate of change of wettability may be measured.

1.3 When using a standard ink, the initial contact angle or wettability is considered to be a measure of the ruling quality of the paper. The rate of change of contact angle or wettability is considered to be a measure of the writing quality.

1.4 Other liquids, both aqueous and nonaqueous, may be used with this technique in special applications. Modification of the procedure may be required when using liquids other than the standard ink described.

1.5 Two types of apparatus for measuring the contact angle are described. Additional apparatus that can perform the test may also provide equivalent results.

1.6 The values stated in inch-pound units are to be regarded as the standard. The values given in parentheses are for information only.

1.7 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

This test method is DOD Approved, ANSI Adopted

D5725-99 Standard Test Method for Surface Wettability and Absorbency of Sheeted Materials Using an Automated Contact Angle Tester

1. Scope

1.1 This test method measures the contact angle of a test liquid in contact with a flat specimen of a film or a paper substrate under specified test conditions. This test method may be used with any liquid of interest which is compatible with the equipment used, particularly with regard to liquid viscosity, tackiness, and vapor pressure (evaporation). This test method may be used with any substrate of interest, which can be cut to dimensions compatible with the equipment used.

1.2 For materials which sorb the test liquid under the specified test conditions, the rate of change of the contact angle as a function of time may be significant, and may be determined using procedures described in this test method. It is also possible to evaluate the sorptive properties of a surface, as the remaining liquid volume on top of the specimen surface is measured as a function of time.

1.3 The conditions required in this test method specify reagent water as the test liquid when testing papers designed to be absorbent, such as absorbent tissue grades.

1.4 Conditions are specified for the testing of a wide range of papers considered to be of low absorbance or nonabsorbent, including release papers, sized, coated, or unsized papers designed for printing, writing, wrapping, and similar tasks where the paper surface interaction with aqueous or solvent based inks or other aqueous or nonaqueous liquids is important. In such cases, test liquids other than reagent water, including writing and printing inks, or organic liquids or mixtures of organic liquids may be used as the test liquid upon prior agreement of those involved in the testing, provided the liquid is compatible with the equipment used. Where test liquids other than reagent water are used, the actual liquid used is reported.

1.5 Conditions are also specified for the testing of polymer films, polymer-coated papers, paper laminates, felt, textiles and non-wovens, using water or other fluids compatible with the equipment and important to the end-use applications of the materials tested, including gluing and printing.

1.6 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

C813-90(1994)e1 Standard Test Method for Hydrophobic Contamination on Glass by Contact Angle Measurement

1. Scope

1.1 This test method covers the detection of hydrophobic contamination on glass surfaces by means of contact angle measurements. When properly conducted, the test will enable detection of fractions of monomolecular layers of hydrophobic organic contaminants. Very rough or porous surfaces may significantly decrease the sensitivity of the test.

1.2 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

D5946-96 Standard Test Method for Corona-Treated Polymer Films Using Water Contact Angle Measurements

1. Scope

1.1 This test method covers measurement of the contact angle of water droplets on corona-treated polymer film surfaces, with subsequent estimation of the film's wetting tension.

Note 1- There is currently no ISO standard that duplicates this test method.

1.2 The values stated in SI units are to be regarded as the standard. The values given in parentheses are for information only.

1.3 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.

For more information on surfactants, please visit the Surfactants Virtual Library at www.surfactants.net