When water hits a surface, it can either spread out into a thin puddle or bead up into a tight droplet. This interaction is quantified by a measurement known as the water contact angle. It is the angle formed at the edge of a droplet, precisely where the liquid meets the solid surface. On one surface, a drop might flatten and form a low angle at its edge, while on another it might form a tight ball with a large angle. This angle is a direct visual indicator of the forces between the water and the surface.
Understanding Surface Wettability
A water droplet’s shape is determined by a balance between two forces: cohesion and adhesion. Cohesion describes the intermolecular forces that cause water molecules to stick to each other. Adhesion describes the forces that attract water molecules to the solid surface. The contact angle is the physical manifestation of this interplay.
If the adhesive forces between water and a solid are stronger than the cohesive forces within the water, the droplet will spread out. This action is known as wetting. Surfaces that promote this are called hydrophilic, or “water-attracting,” and have a water contact angle less than 90 degrees. A clean glass slide is a common example, as water spreads across it easily.
Conversely, if cohesive forces are stronger than adhesive forces, the water will resist spreading and form a distinct bead. This characterizes a hydrophobic, or “water-repelling,” surface, where the contact angle is greater than 90 degrees. A freshly waxed car provides a clear illustration, as water beads up into tight droplets that roll off easily.
When this water-repelling property is pronounced, the contact angle exceeds 150 degrees, and these surfaces are classified as superhydrophobic. On such a surface, a water droplet rests on microscopic air pockets trapped by the surface’s texture, barely making contact with the solid material. This results in an extreme beading effect, allowing water to roll off with the slightest tilt.
How Water Contact Angle Is Measured
Measuring the water contact angle provides a quantitative value for surface wettability. The most common instrument is an optical tensiometer, or goniometer. This device captures the profile of a liquid droplet on a solid substrate to determine the angle formed where the liquid, solid, and air meet.
The standard procedure is the sessile drop method. It begins with placing a precise droplet of a liquid, like purified water, onto the flat surface being tested. A high-resolution camera captures a profile image of the droplet. Specialized software then analyzes this image, identifies the baseline of the drop, and fits a mathematical expression to its shape to measure the angle at the point of contact.
This method provides a static contact angle, which is the measurement of a droplet at rest. For more advanced analysis, scientists can also measure dynamic contact angles. These include the advancing angle, measured as the droplet volume is increased just before the contact line moves outward, and the receding angle, measured as volume is withdrawn. The difference between these two angles, known as contact angle hysteresis, provides deeper insights into factors like surface roughness and chemical heterogeneity.
Contact Angle in Nature and Technology
The principles of contact angle are widely observed in nature and harnessed in technology. Nature is filled with surfaces that have evolved to control their interaction with water, like the superhydrophobic lotus leaf. This “Lotus effect” causes water to bead into spheres that roll off easily, collecting dirt and cleaning the plant.
Engineers have mimicked these natural designs to create a wide array of useful products. Waterproof and breathable fabrics, like Gore-Tex, feature a membrane with pores large enough to allow water vapor to escape but too small for liquid water to penetrate. Non-stick coatings on cookware, like Teflon, use a low-energy surface chemistry that creates a high contact angle with water and oils, preventing food from adhering.
The manipulation of surfaces to be hydrophilic is also important in technology. Anti-fog coatings for eyeglasses and vehicle windshields work by creating a surface with a very low contact angle. This causes moisture to spread into a thin, uniform sheet rather than forming light-scattering droplets that obscure vision. In the medical field, some biomedical implants are designed with hydrophilic surfaces to encourage integration with bodily tissues, while others are made hydrophobic to resist bacterial colonization.
What Changes the Water Contact Angle?
A surface’s contact angle is not an immutable property; it can be modified by altering its physical and chemical characteristics. The two primary factors are its roughness and chemical composition. These elements can be fine-tuned to enhance or reduce the wettability of a material.
Surface roughness plays a significant part in determining the contact angle. For a surface that is already hydrophobic, increasing its roughness can amplify this effect, leading to higher contact angles. A rougher texture can trap air beneath the water droplet, reducing the contact area between the liquid and the solid, a principle observed in surfaces like the lotus leaf.
The most direct way to alter a contact angle is by changing the surface chemistry. Applying chemical coatings or treatments can transform a surface’s properties. For instance, a hydrophilic material like glass can be made hydrophobic by applying a waxy or silicone-based substance. Conversely, industrial processes like plasma treatments are used to increase the surface energy of plastics, making them more hydrophilic so that inks and coatings adhere properly.