The wetting behavior of a liquid on a solid surface is a phenomenon of significant practical importance. The angle of liquid-to-solid contact is important in areas such as adhesion, adsorption, lubrication, catalysis, solid-liquid reaction kinetics, heat transfer, electrical conduction, and microfluidic devices. The angle of contact is one way to measure and assess the phenomenon of liquid wetting of a solid surface.
The contact angle of a liquid on a surface may be used to define to what extent, if any, a liquid will “wet” or contact a surface. Whenever a liquid contacts a solid surface, several different types of behavior can be exhibited. At one extreme, a drop of liquid contacting a solid surface will spread out until it forms a thin film on the surface. This is called total wetting and in this case, the liquid has a contact angle of zero with the surface. At the other extreme, a drop of liquid will sit on the surface like a marble with minimal contact. This behavior is termed non-wetting, and the liquid in this case forms a contact angle of 180 degrees with the surface. For situations in between these extremes, a drop will be formed that makes a well-defined contact angle with the surface. This is called partial wetting.
Because the wettability of liquids on solid surfaces is important to quantify, there have been many approaches used to measure the contact angle of a liquid on a solid surface. Prior art measurement approaches have included the sessile drop method, the tilting plate method, the Wilhelmy plate method, and the capillary rise method. Typically, the wettability of a surface is determined largely by the intrinsic contact angle that the liquid makes with the solid surface. The tilting plate method may be difficult to perform if only small amounts of liquid are available. The other methods typically require expensive goniometer-mounted telemicroscopes to accurately view the contact angle optically. These techniques may also have difficulty measuring the contact angle to an accuracy of better than 5 degrees. These techniques may also have difficulty measuring dynamic changes in the contact angle. In addition, some of these techniques require expensive computer software to analyze the liquid interface and obtain a desirable accuracy of one percent in the measurement of contact angle.
With the expanded need to measure the contact angle of liquids on various surfaces, there is a need for a rapid and inexpensive means to measure both the static and dynamic contact angles of liquids on solid surfaces to one percent accuracy that will be accessible to any size laboratory, institution, or business.
The Transition Angle Method (TAM) is applicable to both wetting and non-wetting liquids in static and dynamic situations. This accurate, yet simple technique allows contact angle measurements to be conducted in situations previously not feasible, such as when low cost and/or portability are highly desired.
Today, contact angles of liquids on solid surfaces are measured by examining drops under high-power magnification. This process often requires sophisticated equipment and specialized technicians. In TAM, an angular feature is created from the solid material in question, and the contact angle is determined indirectly by observing at what angle the liquid fills the vertex (Figure 1). The transition angle at which the liquid wets the vertex completely can be used to readily calculate the contact angle of the liquid on the material in question. The angular features can be dynamically created with a hinged surface or with a series of preformed, progressively wider angled grooves (Figure 2). The TAM method is much easier to accomplish than observing the contact angle directly.