The familiar phenomenon of stickiness is fundamentally a story of molecular attraction, a process scientists call adhesion. Adhesion describes the forces that draw two different surfaces together, while cohesion refers to the internal forces that keep a single substance, such as a drop of water or a block of glue, intact. A sticky substance possesses physical or chemical properties that maximize the attractive forces between its molecules and the molecules of another surface. This interaction determines why some materials cling to a wall, why certain creatures can walk on ceilings, and why some substances bond materials together permanently.
The Physics and Chemistry of Adhesion
The ability of a substance to adhere relies on three principal mechanisms that operate at the interface between materials.
Intermolecular Forces
The most pervasive mechanism involves intermolecular forces, specifically the collective action of van der Waals forces. These weak, short-range electrical attractions arise from the temporary fluctuation of electron clouds around atoms, creating transient, induced dipoles that attract nearby molecules. For these forces to generate significant adhesion, the two surfaces must be brought into extremely close proximity, ideally within a few nanometers of each other.
Chemical Bonding
Chemical bonding represents a much stronger form of adhesion, occurring when an adhesive and a substrate form permanent covalent or ionic bonds. This mechanism is characteristic of structural adhesives, such as epoxies, which cure or set to create a rigid, high-strength connection. The process often involves a chemical reaction, requiring heat, moisture, or a curing agent to permanently link the molecules of the adhesive to the substrate.
Mechanical Interlocking
A third mechanism, known as mechanical interlocking, is purely physical and relies on the adhesive flowing into the microscopic pores and irregularities of the surface. Once the adhesive hardens or cures within these voids, it acts like countless tiny grappling hooks, holding the surfaces together by physical obstruction. A highly viscous liquid adhesive is effective at penetrating these microscopic asperities, which is why roughening a surface often improves the final bond strength.
Nature’s Diverse Strategies for Stickiness
Living organisms have evolved highly specialized adhesive systems that manipulate fundamental forces to suit diverse environments.
Dry Adhesion (Geckos)
Geckos utilize a system of dry adhesion that relies entirely on physical forces without secretions. The bottoms of their feet are covered in millions of microscopic hairs called setae, which branch into hundreds of flatter tips known as spatulae. These nanoscale spatulae maximize the surface area in contact with the substrate, allowing the cumulative effect of van der Waals forces to generate immense sticking power. The system is reversible; the gecko achieves disengagement by peeling its toes away at a specific angle, around 30 degrees.
Wet Adhesion (Mussels)
Other animals employ wet adhesion, which is effective in aquatic environments. Mussels, for example, anchor themselves to rocks using strong byssal threads coated with specialized adhesive proteins. These proteins contain high concentrations of the amino acid DOPA, which enables the adhesive to cure and bond effectively even when submerged in saltwater. This underwater chemical bonding strategy allows the mussel to withstand powerful hydrodynamic forces.
Pressure-Sensitive Adhesion (Insects)
Insects like flies and stick insects often use pressure-sensitive adhesion, relying on a thin liquid film secreted from specialized pads. The liquid film enhances adhesion through capillary and viscous forces, wetting the surface to increase the contact area and reduce the effects of surface roughness. In stick insects, research suggests that adhesive performance is enhanced by the application of shear force, meaning they stick better the harder they pull sideways. This shear-sensitive mechanism allows larger insects to maintain secure footing.
When Adhesion Causes Problems
While adhesion is often beneficial, it can also lead to significant problems, particularly in health and industrial settings, a phenomenon known as biofouling. This occurs when microorganisms, such as bacteria, attach to surfaces and form complex, persistent layers called biofilms. A common example is dental plaque, where bacteria anchor themselves to the tooth surface and excrete a sticky matrix of extracellular polymeric substances (EPS).
The EPS matrix, composed of polysaccharides, proteins, and DNA, provides a protective shield. This shield makes the embedded bacteria highly resistant to immune responses and antibiotic treatments. In the medical field, biofilms on devices like catheters and implants are responsible for up to 80% of all human infections.
Industrially, biofouling causes substantial financial losses by reducing efficiency in water systems and manufacturing. The slimy layers accumulate in cooling towers and pipes, leading to reduced heat transfer, blockages, and accelerated corrosion. Because these microbial communities adhere so strongly and resist removal, specialized chemical and physical treatments are often required to manage the problem.
Engineering Controlled Synthetic Adhesives
Human engineering has harnessed the principles of adhesion to create a wide array of synthetic materials for bonding and temporary attachment. Synthetic adhesives are categorized based on their bonding mechanism, primarily falling into reactive or pressure-sensitive types.
Reactive Adhesives
Reactive adhesives, such as two-part epoxies or polyurethane glues, form permanent, high-strength bonds by undergoing a chemical reaction that links the polymer chains together. They are used in structural applications where durability and resistance to environmental factors are required.
Pressure-Sensitive Adhesives (PSAs)
Pressure-sensitive adhesives (PSAs), used in tapes and sticky notes, rely on physical adhesion without a chemical cure. These materials are viscoelastic polymers, meaning they possess both viscous (flowable) and elastic (shape-retaining) properties. The viscous nature allows the adhesive to flow and “wet out” the surface when light pressure is applied, maximizing contact and engaging van der Waals forces. The elastic properties of PSAs allow them to resist internal deformation and maintain their structure when pulled, permitting clean removal. Chemists manipulate the composition of these adhesives, often by adding tackifiers, to finely tune the balance between initial stickiness and ultimate bond strength for specific applications, ranging from industrial tapes to temporary memo pads.