Interfacial tension, Surface Tension, Contact Angle, and Wetting

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Interfacial tension and interfacial rheology measurements can inform on an enormous number of liquid and solid phenomena and processes, such as:

  • Foam and emulsion formation
  • Foam and emulsion stability
  • Wettability and the dynamic wetting of surfaces
  • Atomization and droplet formation
  • Flow in microfluidic and lab-on-a-chip devices
  • Drainage characteristics
  • Coatability
  • Penetration and absorption processes
Interfacial tension can be thought of as a skin that stretches across an interface. That interface can be between two immiscible liquids, such as oil and water, a liquid/gas interface or a liquid/solid interface. Interfacial tension arises from a concentration of cohesive force between like molecules due to a scarcity of neighbouring like molecules compared to the conditions in the (three-dimensional) bulk.  Surface tension is the term generally applied to the specific situation of interfacial tension at an air/liquid interface.

Drop shape analysis takes two approaches:

Pendant drop analysis measures the shape of a liquid drop suspended from the end of a tube in air or a lower density liquid. The shape of the hanging drop of liquid is the result of the competing forces of gravity, trying to pull the drop into a pear shape, and surface tension, trying to maintain a sphere. By analysis of the resulting drop shape surface tension and interfacial tension can be calculated.

  A pendant drop closeup

Sessile drop analysis entails measuring the angle at which a small drop of liquid contacts a surface. Contact angle ranges from 0 degrees for complete wetting of a surface to 180 degrees for complete non-wetting. In the case of water on a surface, a contact angle of less than 90 degrees characterizes the surface as hydrophillic and hydrophobic if the contact angle exceeds 90 degrees. Contact angle measurements are typically used to measure the wettability of a known surface through the use of drops from various liquids under test. Alternatively, by applying known liquids to an unknown material, the surface energy of that material can be quantified.
Sessile drop close up

Dynamic contact angle measurement methods

Dynamic contact angle measurement is the term given to the quantification of advancing and receding contact angle. Advancing contact angle is that associated with wetting fresh, previously un-wetted surface. Receding contact angle is that angle associated with the dewetting of a surface, as the liquid front recedes. The difference between advancing and receding contact angle is know as hysteresis and can inform about the uniformity of a surface treatment, its roughness and cleanliness.
Dynamic contact angle measurement, figure a shows wetting, figure b shows de-wetting.
Dynamic contact angle measurement is performed in two ways: through altering the volume of a sessile drop by dosing liquid to form a maximum advancing angle (Figure a) and removing liquid to form a minimum receding angle (Figure b), or by the use of a tilting table (below) which measures the angles at the front and rear of a drop at the incipient of movement. The tilting table method employed here can also be used to measure the “roll off angle” for approximately spherical drops which provides a simple measure of the draining ability of a treated surface.

Dynamic Wetting Measurement

Wetting is the result effect of the adhesive forces between a solid surface and a liquid drop, and the cohesive forces within the liquid drop. Strong liquid-solid surface cohesive forces minimise the contact angle, while stronger liquid-liquid adhesive forces lead to an increased contact angle. The dynamic wetting measurement can be recorded using high speed imaging to capture the contact angle at rapid intervals subsequent to the placing of a drop. This can inform on rapid adsorption of surface active entities or on situations where significant interaction, either chemical or physical, with a surface may occur. The main commercial applications of this technique are:

  • Determining surface functionality.
  • Development of self-cleansing films, fabrics and glass surfaces.
  • Measuring the spreading rate of various liquids on a surface.

Interfacial Rheology Measurement

Interfacial rheology is an exciting and relatively new technique that enables the characterization of viscoelastic properties of an interface such as modulus and stress relaxation.  These properties arise from the time-dependent interchange of surface active entities between the bulk solution and the interface. As interface is created surfactants take a finite time to adsorb to an equilibrium condition; if the interface is then compressed those surfactants are then forced back into the solution.
By oscillating the size of a pendant drop and measuring the interfacial tension throughout it is possible to characterize these processes. This method is known as dilatational interfacial rheology.
 Oscillating pendant drop shape analysis

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