The performance of many products depends on the characteristic of interactions of liquids with liquids or liquids with solids. Surface tension, wetting, and adhesion are interconnected properties of liquids and impact product performance.

Surface tension is a property of liquids that makes their surface behave like a stretched elastic membrane, due to the cohesive forces between liquid molecules. This means the surface of a liquid resists external forces and tends to minimize its surface area. It’s a common phenomenon that explains things like water droplets forming round shapes, insects walking on water, and the ability of some objects like paper clips to float on the surface despite being denser than water.

Surface tension in combination with surface energy determines how well a liquid will spread (wet) or will not spread (beading) on a solid surface.

Understanding and controlling surface tension allows engineers and product designers to tailor products for specific functionalities, enhance performance, and ensure quality in diverse applications like the following:

• adhesives, paints, and coatings, where proper wetting ensures good adhesion and uniform coverage;
• sealants and barrier coating where proper beading ensures effective barrier properties;
• detergents and cleaning products where proper wetting ensures penetration;
• ink jet printing where the ink’s surface tension affects the formation and deployment of droplets onto the paper
• emulsions where surface tension plays a key role in stability, texture, appearance, and effective delivery of active ingredients
• foams where surface tension plays a key role in stability or the product; and

PAINTS, COATINGS, and ADHESIVES

Liquids wet the surface and spread over it when the attraction of the liquid to the surface is greater than the liquid’s cohesive forces. 

• Paints and Coatings: The surface tension of paints and coatings is adjusted to ensure they spread evenly across surfaces and adhere properly, so that when you paint your living room wall, it is smooth, sticks and looks attractive.
• Adhesives: For adhesives to achieve optimal bonding, the substrate to be bonded must have a surface energy higher than the adhesive’s surface tension, thus achieving good wettability or wetting. Hot melt adhesives must have lower surface tension than surface energy of the substrates to help its wetting

SEALANTS AND BARRIER COATINGS

Liquid droplets form on a surface when the cohesive forces that cause the liquid to minimize its surface area are stronger than the adhesive forces of the surface.

• Surface tension is a key property that is analyzed in the development of protective sealants and barrier coatings:
• Sealants: Protective sealants are applied countertops and flooring are specially formulated so that the various types of liquids, aqueous and non-aqueous, will bead up and not soak into the surface. High surface tension of the liquid and low adhesive forces to the surface are the reason why red wine will bead up on the surface of your countertop, making it easy to wipe up without leaving a stain
• Food Packaging Barrier Coatings: Specialty coating that are safe for contact with food are applied to food packaging materials so that hamburger wrappers preventing grease from soaking through, so you don’t get stains your car upholstery and clothes after you go through the drive-through.

DETERGENTS AND CLEANING PRODUCTS

Laundry detergent lowers the surface tension of water, allowing it to better penetrate fabrics and remove dirt, so you have fresh clean clothes.

• Detergents contain surfactants, which are molecules with both hydrophilic (water-attracting) and hydrophobic (water-repelling) ends.
• These surfactants disrupt the cohesive forces between water molecules, reducing surface tension.
• Lowering surface tension allows water to spread out and wet surfaces more effectively, improving cleaning.

INK JET PRINTING

Surface tension plays a critical role in the formation and ejection of ink droplets, as well as their behavior on the printing surface.

• The surface tension of the ink must be carefully balanced to ensure proper droplet formation, ejection, and wetting of the substrate, ultimately affecting print quality and reliability.

More details about the importance of surface tension in ink jet printing

• Droplet Formation and Ejection: The surface tension of the ink, combined with viscosity and other properties, influences how ink droplets are formed at the print head nozzles and how they are ejected.

• Wetting and Spreading: Once ejected, the ink droplet’s surface tension affects how it interacts with the substrate (e.g., paper). If the surface tension is too high, the ink may bead up or not spread evenly, leading to poor image quality.
• Surface Tension and Print Quality: A well-balanced surface tension allows for the ink to spread evenly on the substrate, resulting in sharp, clear prints. If the surface tension is too low, the ink may spread too much, causing smearing or bleeding.
• Controlling Surface Tension: Ink formulations include additives like surfactants and alcohols to adjust surface tension to the desired level for optimal printing performance.

• Surface Energy and Wettability: The surface energy of the substrate also plays a role. For good print quality, the substrate’s surface energy should be higher than the ink’s surface tension, promoting good wetting and adhesion, says Kao Collins.

EMULSIONS

Surface tension plays a key role in the stability of emulsions and foams, ensuring desired texture, appearance, and effective delivery of active ingredients.

• Emulsions are mixtures of two immiscible liquids, such as oil and water, where one liquid is dispersed as droplets within the other.
• High surface tension between the two liquids makes them resistant to mixing. Surfactants, which are molecules with both hydrophilic (water-attracting) and hydrophobic (oil-attracting) parts, adsorb at the interface between the liquids, reducing the surface tension and allowing for the formation of stable droplets.

Examples of stable emulsions include:

• Paints and Coatings:
  • Paints are essentially emulsions where pigments and other additives are dispersed in a liquid medium, typically water or oil.
• Agricultural Chemicals:
  • Many pesticides and herbicides are formulated as emulsions, where the active ingredient is dispersed in a liquid carrier. This allows for easier application and better coverage of crops.
• Petroleum Industry:
  • Drilling fluids, used in oil and gas exploration, often contain emulsions to help lubricate the drill bit and remove cuttings from the borehole. Emulsions are also used in enhanced oil recovery techniques, where they help to displace oil from reservoirs.
• Food Industry:
  • Mayonnaise: A classic example of a stable oil-in-water emulsion stabilized by egg yolk. The oil droplets are finely dispersed in the water-based mixture.
  • Salad Dressing: Similar to mayonnaise, salad dressings are often emulsions of oil and vinegar, stabilized by emulsifiers or other ingredients.
  • Butter: Another example of a water-in-oil emulsion, where water droplets are dispersed in a continuous fat phase.
  • Milk: A natural emulsion of fat globules dispersed in an aqueous solution.
  • Chocolate: Ganache is essentially a mixture of chocolate and cream, where the cream (water-based) and chocolate (fat-based) are emulsified to create a smooth, pourable, and stable consistency.

FOAMS

Foam is a substance formed by trapping many gas bubbles in a liquid. Foam formation involves reducing the surface tension of a liquid, which lowers the energy needed to create new interfaces between the gas and liquid. 

• A stable foam is one where the bubbles resist merging (coalescence) or draining of liquid from between the bubbles (drainage).

Examples of Stable Foams:

• Shaving cream: Shaving cream relies on surfactants to create a stable foam that allows for a smooth shave.
• Firefighting foam: These foams, often made with synthetic surfactants, are used to create a barrier on fires, preventing oxygen from reaching the fuel.
• Beer: The foam on these drinks is due to dissolved gases and the presence of proteins or other surfactants.
• Personal care products: Shampoos, body washes, and bubble baths use surfactants to create stable foams that enhance cleaning and user experience.

• Some food products: Certain food foams, like whipped cream or meringue, rely on proteins and other ingredients to create stable air pockets.

SUMMARY

In summary, understanding and controlling surface tension allows engineers and product designers to tailor products for specific functionalities, enhance performance, and ensure quality in diverse applications.  SNP’s technical team understands the importance of surface tensions in designing its products for the coatings and adhesives markets.  Contact us to learn more about our specialty products.