Biomaterials Tutorial

Contact Angle

Jeanette Stein
University of Washington Engineered Biomaterials

Contact angle analysis characterizes the wettability of a surface by measuring the surface tension of a solvent droplet at its interface with a homogenous surface.  In more technical terms, contact angle measures the attraction of molecules within the droplet to each other versus the attraction or repulsion those droplet molecules experience towards the surface molecules. These are termed “force balances” (See Figure 1 for an example). Contact angle is one of the most sensitive and inexpensive surface analysis techniques and is capable of measuring ~3-20 Å deep.

There are five common techniques that can be employed to measure the contact angle. These have been summarized in Figure 2.  The technique chosen depends principally on the geometry and location of the surface or coating to be studied. In all the methods, the contact angle (q) is the angle of the liquid at the interface relative to the plane of the model surface. 

Static or Sessile Drop Method

The most commonly used technique is the static or sessile drop method (Fig. 2a). The experiment normally calls for the successive addition of fluid droplets until a plateau in the contact angle is reached. This plateau is known as the “advancing contact angle” (Fig. 3). Immediately following the advancing contact angle experiment, it is useful to obtain a ‘receding contact angle’ value by monitoring the contact angle as equivalent volume droplets of fluid are successively retracted from the droplet. It is important to understand that the advancing and retreating angles are usually not equal (see Figure3). There is normally a high level of experimental hysteresis resulting from sample pre-hydration, surface roughness, chemical heterogeneity, evaporation and/or molecular movement. Thus, the receding contact angle allows one to measure the degree of hysteresis inherent in the sample surface.

Wilhemly Plate Method

The Wilhemly plate method (Fig. 2b) is ideal for double-sided samples that need to be tested in temperature-controlled conditions. Certain surfaces may be temperature-sensitive; they are hydrophobic at one temperature and hydrophilic at another. The temperature of a beaker of water is easier to monitor and maintain at a constant temperature than the temperature of fluid droplet. So, the contact angle method has been altered for those samples that need a higher level of control.

Captive Air Bubble Method

An alternative to the Wilhemly plate method is the captive air bubble method (Fig. 2c). In this method, the contact angle is measured between an air bubble of defined volume and the solid surface immersed in the temperature controlled bath.

Capillary Rise Method

The capillary rise method (Fig. 2d) presents the only method of contact angle measurement available for the measurement of tubular materials and coatings. Temperature may be maintained in this method over a short period of time.

Tilted-drop Measurement

The tilted-drop measurement (Fig. 2e) is another angle measurement. In this technique, a droplet is added to the surface and the advancing and retreating contact angle are measured as the surface is tilted up until the droplet reaches a point where it almost moves. This technique is useful to measure both the receding and advancing contact angles at the same time.

In general, contact angle measurements serve as a good initial technique to characterize a surface. However, contact angle measurements need to be analyzed with care as a number of factors including operator error, surface roughness, surface heterogeneity, contaminated fluids, and sample geometry can influence the overall result.

Figure 1  Figures 1A and 1B demonstrate a difference in wettability. Figure 1A shows how a water droplet might appear on a hydrophobic surface such as wax. Figure 1B shows how a water droplet might appear on a hydrophilic surface such as a contact lens

Figure 2.   Five ways that the contact angle (q) can be measured. (A.) Sessile or Static drop. (B.) Wilhelmy plate method. (C.) Captive air bubble method. (D.) Capillary rise method. (E.) Tilting substrate method. Figure adapted from Ratner, et. al.

Figure 3. A generalized contact angle plot showing the advancing (qAdv) and receding (qRec) contact angles.

References:

1. Biomaterials science: An introduction to materials in medicine. Ratner BD, Hoffman AS, Schoen FJ, Lemons JE, editors. San Diego, CA: Academic Press, 1996.
2. Ulman A.  An introduction to ultrathin organic films: From Langmuir-Blodgett to self-assembly. Boston, MA: Academic Press, 1991.
3. Neumann AW, Godd RJ. Techniques of measuring contact angles. In: Good RJ, Stromberg RR, editors.  Surface and colloid science—Experimental methods, vol. 11. New York: Plenum Publishing; 1979. pp. 31-61.