Surface Energy vs. Tension: The Key Differences and Examples

Surface energy and surface tension are both about how molecules stick together at surfaces. 

Surface energy is mostly about solids, like how a sticker sticks to a wall. Surface tension is for liquids, like how water forms droplets. Even though one is for solids and the other for liquids, they have a lot in common and play big roles in our daily lives.

Here we’ll study both the differences and similarities between surface energy and surface tension.

Surface Energy vs. Surface Tension

What is Surface Energy?

Surface energy refers to the energy required to increase the surface area of a solid by a unit amount. In essence, it quantifies the disrupted intermolecular bonds when a new surface is created.

The concept of surface energy stems from the cohesive forces between molecules. Just as molecules within a solid attract each other, molecules on the surface are not fully satisfied in terms of their bonding capability, leading them to possess excess energy.

This excess energy of surface molecules, as compared to those in the bulk of the material, is defined as surface energy.

Examples:

• Adhesion of Paint: The ability of paint to adhere well to a surface is influenced by the surface energy of the material being painted. High surface energy materials, like metals, often allow for better paint adhesion than low surface energy materials, like certain plastics.
• Wettability of Surfaces: The interaction of water droplets on surfaces, whether they spread out or bead up, is determined by the surface energy of the material. A high surface energy results in better wettability.

How Surface Energy Can Be Measured

Surface energy is often determined by evaluating the wettability of a solid, which in turn is assessed through contact angle measurements:

Contact Angle Measurement: When a liquid droplet is placed on a solid surface, the angle formed between the solid surface and the tangent to the droplet at its point of contact is the “contact angle”. A smaller contact angle indicates a higher surface energy, while a larger contact angle points to a lower surface energy. The Young’s equation is often used to relate the contact angle to the surface energy.

Dyne Pens/Test Inks: These are liquids with known surface tensions. When applied to a solid, if the liquid spreads out, it indicates the surface energy of the solid is higher than the liquid’s surface tension. Conversely, if it beads up, the solid has a lower surface energy.

Factors Affecting Surface Energy:

• Material Composition: Different materials inherently possess different surface energies. For instance, metals generally have higher surface energies than polymers.
• Roughness: The microstructure and roughness of a surface can influence its effective surface energy. Rough surfaces often have increased surface areas which can impact the perceived surface energy.
• Chemical Treatments: Surfaces can be modified with treatments or coatings to change their surface energy. For example, corona or plasma treatment can increase the surface energy of polymers.
• Environmental Factors: Exposure to air, moisture, or other environmental factors over time can change the surface energy of materials.
• Temperature: As temperature changes, molecular motion and vibrations change, which can affect the surface energy of a material.

What is Surface Tension?

Surface tension is the property of a liquid that allows it to resist external forces. It is the force that acts at the surface of a liquid, opposing its expansion and minimizing its surface area. This phenomenon can be attributed to the cohesive forces between the liquid molecules.

Molecules within the liquid are surrounded by other molecules on all sides, and they experience balanced attractions. However, molecules on the surface have no molecules above them and hence experience a net inward force.

This results in the liquid behaving as if its surface is covered by a stretched elastic membrane.

Examples:

• Water Droplets: The formation of spherical water droplets on a surface or in the air is a result of surface tension. The liquid tends to minimize its surface area, leading to the formation of a droplet.
• Capillary Action: Surface tension allows water to rise against gravity in narrow tubes or in the soil, which is crucial for plant life.

How Surface Tension Can Be Measured

The measurement of surface tension is often based on the forces exerted by the liquid in response to external objects:

Drop Weight Method: By measuring the weight of droplets dropping from a tip, one can determine the surface tension. The liquid detaches when its weight overcomes surface tension forces.

Wilhelmy Plate Method: Immersing a flat, thin plate into a liquid vertically and measuring the force required as the plate is wetted provides a measure of surface tension.

Maximum Bubble Pressure Method: By measuring the pressure inside a bubble as it’s being formed and before it detaches, the surface tension of the liquid can be calculated.

Factors Affecting Surface Tension

• Impurities: The presence of impurities or solutes in the liquid can alter its surface tension. For example, soaps and detergents reduce the surface tension of water, allowing it to spread more easily.
• Temperature: Surface tension generally decreases with increasing temperature. As the temperature rises, the kinetic energy of the molecules increases, reducing the cohesive forces between them.
• Nature of Liquid: Different liquids inherently have different surface tensions due to the nature of their intermolecular forces.
• Presence of Surfactants: Surfactants can significantly reduce the surface tension of a liquid by accumulating at the surface and disrupting the cohesive forces between the liquid molecules.
• Electrical Forces: External electric fields can influence the arrangement of molecules at the surface of a liquid, thereby affecting its surface tension.

Overview of the Key Differences Surface Energy and Surface Tension

Similarities Between Surface Energy and Surface Tension

Cohesive Forces: Both surface energy and surface tension arise due to cohesive forces between molecules. In surface energy, it’s the cohesive forces of molecules on a solid surface, while in surface tension, it’s the cohesive forces between liquid molecules at the surface.

Minimization of Energy: Both phenomena inherently aim to minimize energy. Surface energy reflects the energy of the surface atoms or molecules in the solid, and a material will tend to reduce its total surface area to minimize this energy. Similarly, surface tension leads a liquid to reduce its exposed surface area, such as forming droplets.

Measurement by Contact Angle: Both can be related to the contact angle of a liquid on a surface. A liquid will wet a high surface energy surface, leading to a low contact angle, and the liquid itself will exhibit a certain contact angle based on its surface tension.

Influence of Temperature: Both surface energy and surface tension are influenced by temperature. Typically, as temperature increases, the surface energy of solids and the surface tension of liquids both decrease.

Importance in Wetting: The wetting behavior of a liquid on a solid is influenced by both the surface energy of the solid and the surface tension of the liquid. Complete wetting or spreading occurs when the surface energy of the solid is close to or exceeds the surface tension of the liquid.

Applications in Industry: Both surface tension and surface energy have significant industrial applications. Whether it’s in designing coatings, manufacturing processes, or creating pharmaceutical formulations, understanding these properties is crucial.

Influence of Additives: The presence of additives, impurities, or surfactants can modify both surface tension and surface energy. For instance, surfactants can reduce the surface tension of a liquid and can also alter the surface energy of a solid upon adsorption.