Adhesiveness describes the capacity of one substance to cling to another. This property is present in many aspects of the world around us, from interactions in nature to many human-made items. Understanding how things stick together is a basic concept in materials science, influencing everything from the strength of structures to the effectiveness of everyday products.
How Things Stick
Materials adhere through several fundamental scientific principles. One common mechanism is mechanical interlocking, where an adhesive penetrates small pores or irregularities on a surface, creating a physical anchor. Like Velcro hooks engaging loops or a dovetail joint, liquid adhesives flow into these surface features before drying or curing. This increases contact area and physically impedes separation.
Another way substances adhere is through adsorption, which involves intermolecular forces between molecules at the interface of the two materials. These forces can be strong primary bonds, like covalent or ionic bonds, where electrons are shared or exchanged. Weaker secondary forces, such as van der Waals forces, hydrogen bonding, and dipole-dipole interactions, also contribute to adhesion by creating attractions between molecules. For strong adsorption to occur, the adhesive must thoroughly wet the surface, allowing for intimate molecular contact.
Diffusion is a mechanism particularly relevant for similar materials, especially polymers. It involves the intermingling of polymer chains across the interface, forming an entangled layer that bonds the adhesive and the substrate. Chemical bonding, the strongest mechanism, involves the formation of new, strong chemical bonds between the adhesive and the material it is adhering to. This molecular-level interaction adds to the strength of many adhesives, such as epoxies used to bond aluminum.
Nature’s Adhesives
The natural world offers many examples of adhesiveness. Geckos, for instance, can cling to various surfaces, even smooth ones like glass, using millions of microscopic hair-like structures called setae on their toe pads. Each seta branches into hundreds of smaller spatular tips, which maximize contact with the surface. These spatulae interact with the surface primarily through van der Waals forces, which, when multiplied by millions of contacts, generate a powerful adhesive force.
Mussels and barnacles exhibit strong underwater adhesion. Mussels secrete specialized proteins, including L-3,4-dihydroxyphenylalanine (DOPA), which allows them to bond to wet surfaces like rocks and glass. DOPA can form strong, reversible coordination bonds with inorganic surfaces and, when oxidized, can create covalent bonds with organic surfaces, contributing to their underwater stickiness. Barnacles also produce a strong cement, allowing them to permanently attach to ship hulls and other submerged structures.
Spider silk, particularly the capture silk of orb-weaving spiders, demonstrates strong adhesive properties for trapping prey. This silk is coated with a sticky, aqueous glue that forms uniformly distributed droplets along the thread. This glue, composed of glycoproteins, salts, and fatty acids, adheres to prey through a combination of physical and chemical interactions. The silk’s elasticity also plays a role, allowing the glue droplets and silk thread to deform and absorb the prey’s kinetic energy, enhancing capture efficiency.
Plant seeds and pollen use sticky surfaces for dispersal and adhering. Pollen grains have a viscous, lipid-rich substance called pollenkitt on their surface, which aids adhesion to pollinators and flowers. The adhesion of pollen to the stigma, the female part of a flower, can increase over time, ensuring successful pollination. This initial attachment is primarily due to physical attractions, such as van der Waals forces and capillarity, with the stigma’s microstructure also aiding grip.
Adhesives in Our Daily Lives
Adhesives are widely used in human technology and daily life, offering bonding solutions for many applications. Everyday glues and tapes employ different chemical compositions to create bonds. White glues are polyvinyl acetate (PVAc) polymer dispersions that harden as water evaporates. Superglues, or cyanoacrylates, cure rapidly when exposed to moisture on surfaces, forming strong, rigid bonds. Pressure-sensitive tapes commonly use acrylic or rubber-based adhesives, which stick instantly upon light pressure due to their sticky nature.
Medical adhesives are designed for biocompatibility and specific properties, used in healthcare. Bandages and wound dressings use silicone or acrylic adhesives. Silicone adhesives are gentle and minimize skin trauma upon removal, for sensitive skin or frequent dressing changes. Acrylic adhesives provide strong, long-lasting adhesion for long-term attachment, such as post-surgical care. Surgical glues and dental bonding agents, based on cyanoacrylates or light-curable acrylics, allow for wound closure without sutures or strong bonds between dental materials and tooth structure.
In construction and manufacturing, adhesives are preferred over traditional fastening methods like welding or mechanical fasteners. Epoxy and polyurethane adhesives are widely used for structural bonding, such as anchoring bolts, reinforcing concrete elements, or assembling automotive components. These adhesives can bond dissimilar materials, distribute stress more evenly across a joint, and contribute to lighter designs in vehicles and aircraft. Strong, durable bonds in various environments make them valuable in building materials and electronics assembly.
Coatings and films also use adhesion. Paints, for instance, adhere to surfaces through a combination of mechanical interlocking, where the paint penetrates surface irregularities, and adsorption, involving intermolecular forces. Non-stick coatings, like those found on cookware, are designed to minimize adhesion to food particles. Protective films applied to screens or surfaces adhere securely to prevent scratches or damage, showing how controlled adhesion creates durable and functional layers.
Measuring Adhesion
Measuring adhesion ensures product quality, aids development, and verifies safety. Different applications require specific levels and types of adhesive strength. Common methods for quantifying adhesive strength include peel strength and shear strength tests.
Peel strength measures the force required to separate an adhesive from a surface in a peeling motion. This test is used for flexible materials like tapes and labels. Shear strength quantifies the force needed to slide an adhesive off a surface. Tensile strength, which measures resistance to pulling forces perpendicular to the bond, is also a widely used measurement. These tests help engineers and manufacturers select the most appropriate adhesive for a given task.