CONTACT ANGLES AND WETTING

In food systems, we are often interested in the ability of a liquid to spread over or “wet” the surface of another material. In some situations, it is desirable for a liquid to spread over a surface (e.g., when coating a food with an edible film), while in other situations it is important that a liquid does not spread (e.g., when designing waterproof packaging). When a drop of liquid is placed on the surface of a material, it may behave in a number of ways, depending on the nature of the interactions between the various types of molecules present. The two extremes of behavior that are observed experimentally are outlined below (Figure 5.8):

1. Poor wetting. The liquid gathers up into a lens, rather than spreading across the surface of a material.

2. Good wetting. The liquid spreads over the surface of the material to form a thin film, which has a liquid–gas interface and a liquid–solid interface.

The situation that occurs in practice depends on the relative magnitude of the interactions between the various types of molecules involved (i.e., solid–liquid, solid–gas, and liquid– gas). A system tends to organize itself so that it can maximize the number of favorable

FIGURE 5.8 The wetting of a surface by a liquid depends on a delicate balance of molecular interactions among solid, liquid, and gas phases.

interactions and minimize the number of unfavorable interactions between the molecules. Consider what may happen when a drop of liquid is placed on a solid surface (Figure 5.8). If the liquid remained as a lens, there would be three different interfaces: solid–liquid, solid– gas, and liquid–gas, each with its own interfacial or surface tension. If the liquid spread over the surface, there would be a decrease in the area of the solid–gas interface but an increase in the areas of both the liquid–gas and solid–liquid interfaces. The tendency for a liquid to spread therefore depends on the magnitude of the solid–gas interactions (γSG) compared to the

magnitude of the solid–liquid and liquid–gas interactions that replace it (γSL + γLG). This

situation is conveniently described by a spreading coefficient, which is defined as (Hunter 1993):

S = γ_SG −(γ_SL + γ_LG) (5.6)

If the energy associated with the solid–gas interface is greater than the sum of the energies associated with the solid–liquid and liquid–gas interfaces (γSG>γSL + γLG), then S is positive

and the liquid tends to spread over the surface to reduce the energetically unfavorable contact area between the solid and the gas. On the other hand, if the energy associated with the solid– gas interface is less than that associated with forming the solid–liquid and liquid–gas inter- faces (γSG<γSL + γLG), then S is negative and the liquid tends to form a lens.

The shape of a droplet can be predicted by carrying out a force balance at the point on the surface where the solid, liquid, and gas meet (Figure 5.9) using the Young equation (Hiemenz 1986):

γSG = γSL + γLG cosθ (5.7)

cosθ γ γ

γ

= SG − SL

LG

(5.8)

Here, θ is known as the contact angle, which is the angle of a tangent drawn at the point where the liquid contacts the surface. By convention, this angle is measured from the side of the droplet (Figure 5.9). The shape of a droplet on a surface can therefore be predicted from a knowledge of the contact angle: the smaller θ, the greater the tendency for the liquid to spread over the surface.

So far, we have only considered the situation where a liquid spreads over a solid surface, but similar equations can be used to consider other three-component systems, such as a liquid spreading over the surface of another liquid (e.g., oil, water, and air) or a crystal at an interface between two other liquids (e.g., a fat crystal at an oil–water interface). The latter case is important when considering the nucleation and location of fat crystals in oil droplets and has a pronounced influence on the stability and rheology of many important food emulsions, including milk, cream, butter, and whipped cream (Walstra 1987, Boode 1992, Dickinson and McClements 1995).

The above equations assume that the materials involved are completely insoluble in each other, so that the values of γSG, γSL, and γLG (or the equivalent terms for other three-component

systems) are the same as those for pure systems. If the materials are partially miscible, then the interfacial tensions will change over time until equilibrium is reached (Hunter 1993). The solubility of one component in another generally leads to a decrease in the interfacial tension. This means that the shape that a droplet adopts on a surface may change with time (e.g., a spread liquid may gather into a lens, or vice versa, depending on the magnitude of the changes in the various surface or interfacial tensions).

The contact angle of a liquid can conveniently be measured using a microscope, which is often attached to a computer with video image analysis software (Hunter 1986). A droplet of the liquid to be analyzed is placed on a surface and its shape is recorded via the microscope. The contact angle is determined by analyzing the shape of the droplet. The advantages and disadvantages of a variety of other techniques available for measuring contact angles have been considered by Hunter (1986).

The concepts of a contact angle and a spreading coefficient are useful for explaining a number of important phenomena which occur in food emulsions. The contact angle deter- mines the distance that a fat crystal protrudes from the surface of a droplet into the surround- ing water (Boode 1992) and whether nucleation occurs within the interior of a droplet or at the oil–water interface (Dickinson and McClements 1995). It also determines the amount of liquid which is drawn into a capillary tube and the shape of the meniscus at the top of the liquid (Hunter 1986, Hiemenz 1986). A knowledge of the contact angle is also often required in order to make an accurate measurement of the surface or interfacial tension of a liquid (Couper 1993).