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Wetting

Also known as
wetting agent, spreading, substrate wetting

Wetting is the ability of a liquid to spread over and stay in contact with a solid surface. It depends on the balance between the liquid's attraction to the surface and to itself, and it is usually measured by the contact angle a droplet makes with the surface.

What is wetting?

Wetting describes how a liquid behaves when it touches a solid. A liquid that wets a surface well spreads out into a thin film; one that wets poorly pulls itself into beads. Water on clean glass wets; water on a waxed car or a waxy leaf beads up and runs off.

What decides the outcome is a competition between two attractions: the liquid’s attraction to the solid (adhesion) and the liquid’s attraction to itself (cohesion, expressed as its surface tension). When adhesion wins, the liquid spreads.

How wetting is measured

The standard measure of wetting is the contact angle: the angle, measured through the liquid, between the solid surface and the edge of a droplet resting on it.

  • A contact angle near 0° means the liquid spreads completely.
  • An angle below 90° means the surface is wetted, at least partly.
  • An angle above 90° means the liquid tends not to wet the surface, which is how water behaves on water-repellent materials.

Young’s equation relates the contact angle to three interfacial tensions: between solid and vapor, solid and liquid, and liquid and vapor. It shows why lowering the liquid’s surface tension, or the tension between the liquid and the solid, reduces the contact angle and improves wetting.

What a wetting agent does

A wetting agent is a surfactant added to help a liquid wet a surface. It works by adsorbing at the liquid’s surface and at the liquid-solid boundary, lowering both tensions so the liquid spreads.

Speed matters as much as the final result. When a spray droplet hits a leaf or a coating is brushed onto a surface, the new surface exists for only a fraction of a second. A wetting agent has to reach that surface quickly to help, which is why formulators look at dynamic surface tension, measured on freshly created surfaces, and not only at equilibrium values.

Why wetting matters

  • Agricultural sprays. Leaf surfaces are covered with a waxy cuticle that repels water. Spray adjuvants improve wetting and spreading so that droplets stay on the leaf and cover more of it.
  • Paints and coatings. A coating must wet the substrate to adhere and level evenly. Poor wetting causes defects such as cratering, and the liquid must also wet pigment particles for good pigment dispersion.
  • Cleaning. A cleaning solution has to wet a soiled surface and penetrate the soil before it can lift it, the first step in detergency.
  • Heap leaching. In heap leaching, solution must wet and spread through crushed ore; poorly wetted zones are bypassed.
  • Soils. Some soils become water-repellent, a condition called soil water repellency, and water runs off or channels through them instead of soaking in. Soil surfactants restore wetting.

Wetting and wettability

The two words are related but used differently. Wetting usually refers to how a particular liquid spreads on a surface, the concern in formulation. Wettability is used in reservoir engineering for a rock’s preference to be in contact with water or with oil when both are present, which controls how fluids move through the rock.

Frequently asked questions

A wetting agent is a surfactant used to help a liquid spread across and penetrate a surface. It lowers the surface tension of the liquid and the tension between the liquid and the solid, reducing the contact angle.

The lower the contact angle, the better the wetting. Angles below 90° indicate that the liquid wets the surface; angles approaching 0° indicate that it spreads into a film.

Wetting describes how a liquid spreads on a solid, and is the usual term in formulation. Wettability is the reservoir engineering term for whether rock prefers contact with water or with oil, which governs how the two fluids are distributed and how easily oil can be recovered.

  • Contact angle

    Contact angle is the angle, measured through the liquid, where the edge of a liquid droplet meets a solid surface. It is the standard measure of wetting: low angles mean the liquid spreads, and angles above 90 degrees mean the surface repels it.

  • Surface tension

    Surface tension is the energy per unit area, or force per unit length, at the surface of a liquid where it meets a gas such as air. It arises because molecules at the surface have fewer like neighbors to attract them than molecules inside the liquid.

  • Wettability

    Wettability is the tendency of a rock surface to be in contact with one fluid in preference to another when both are present, usually water or oil. Reservoir rock is described as water-wet, oil-wet, intermediate or mixed-wet, and its wettability controls how oil and water move through it.

  • Adjuvant

    An adjuvant is an ingredient added to a spray mixture, or built into a formulation, to improve how a crop protection or nutrient product performs or how the mixture handles. Surfactants, crop oils, drift reducers and water conditioners are common types.

  • Surfactant

    A surfactant, short for surface-active agent, is a compound that lowers the surface tension of a liquid, or the interfacial tension between two phases, by adsorbing at the boundary between them. Each molecule has a water-attracting head and a water-repelling tail.

  • Pigment dispersion

    Pigment dispersion is the process of breaking up clumps of dry pigment powder and distributing the individual particles evenly and stably in a liquid such as a paint or ink. It involves wetting the pigment, breaking apart agglomerates and stabilizing the particles against re-clumping.

  • Soil surfactant

    A soil surfactant, also called a soil wetting agent, is a surfactant applied to soil or growing media to help water enter, spread and move through it more evenly. Soil surfactants are used to overcome water repellency in turf, horticultural media and some agricultural soils.

Sources

  1. wetting; contact angle, IUPAC Compendium of Chemical Terminology (Gold Book)
  2. Capillarity and Wetting Phenomena, P.-G. de Gennes, F. Brochard-Wyart and D. Quéré, Springer, 2004
  3. Surfactants and Interfacial Phenomena, 4th edition, M. J. Rosen and J. T. Kunjappu, Wiley, 2012