Wettability is the determining factor in many applications, from coatings and medical implants to pesticides and enhanced oil recovery. Because these applications involve such different sample types, contact angle can be measured in several different ways — and the right method depends mainly on your sample and what you need to know about it.
Which method should you use?
|
Method
|
Best for
|
Static or dynamic CA
|
Typical instrument
|
| Sessile drop |
Most solid surfaces, quick QC checks |
Static |
Optical tensiometer |
| Needle method |
Superhydrophobic and smart surfaces |
Dynamic (advancing/receding) |
Optical tensiometer |
| Tilting method |
Smart surfaces, roll-off angle |
Dynamic (advancing/receding) |
Optical tensiometer |
| Captive bubble |
Contact lenses, hydrogels, immersed systems, EOR |
Static or dynamic |
Optical tensiometer |
| Wilhelmy plate |
Fibers down to 7µm |
Dynamic (advancing(receding) |
Force tensiometer |
| Meniscus |
Fibers down to 100µm |
Dynamic (receding) |
Optical tensiometer |
| Washburn |
Powders, porous materials |
Dynamic (advancing), via mass uptake over time |
Force tensiometer |
Sessile drop method
Sessile drop is the most widely used method for measuring contact angle. It's done with an optical tensiometer, ranging from manual instruments to fully automated systems: a droplet, typically water, is placed on the solid sample, a high-resolution camera captures its image, and the software calculates the contact angle automatically.
The result is a static contact angle for the surface. As a rule of thumb, a water contact angle below 90 degrees indicates a hydrophilic surface, while above 90 degrees indicates a hydrophobic one.
Because it's quick and non-destructive, sessile drop measurement is a common choice for quality control, and it's also widely used for surface free energy calculations from multiple probe liquids. Powders are commonly measured this way too, compressed into a tablet to give a flat, measurable surface.
Typical applications: surface free energy calculations, quality control, surface treatment optimization, powders (compressed into tablets)
Read more: How to utilize contact angles in surface characterization: Static contact angles
Advancing and receding contact angles with the needle method
The so-called needle method measures advancing and receding contact angles. A needle is brought close to the sample surface, and liquid — typically water — is pumped in to gradually grow the drop. As the drop's baseline starts to move, the advancing contact angle is measured. The liquid is then withdrawn, shrinking the drop, and the receding contact angle is measured as the baseline pulls back.
Measuring both advancing and receding angles is generally good practice for characterizing a surface's wetting behavior, and the needle method is especially useful for studying superhydrophobic and other smart surfaces.
Typical applications: superhydrophobic and -oleophobic surfaces
Read more: Advancing and receding contact angles for full wetting characterization
Tilting method
The tilting method also measures advancing and receding contact angle, but by tilting rather than by growing and shrinking the drop. A drop is placed on the surface, then the sample stage — or the whole instrument — is gradually tilted until the drop starts to slide. The advancing angle is measured at the front of the moving drop and the receding angle at the back. The method also gives you the roll-off angle: the tilt angle at which the drop begins to move.
Typical applications: smart surfaces, roll-off angle measurement
Captive bubble method
In the captive bubble method, the sample stays immersed in liquid — usually water — throughout the measurement, and contact angle is measured using an air bubble or a liquid less dense than water, such as oil. This makes it well suited to solid-liquid-liquid systems, such as oil-reservoir wettability studies, and to samples that need to stay immersed or hydrated to give a meaningful result, such as contact lenses. On a contact lens, for example, an air bubble is used to determine surface hydrophilicity while the lens stays fully hydrated, closer to how it behaves in actual use.
Typical applications: contact lenses, superhydrophilic surfaces, enhanced oil recovery
Read more: How to study the wettability of the contact lens?
Wilhelmy plate method
Wilhelmy plate is best known as a surface tension method, but the same setup measures contact angle just as well. It's done with a force tensiometer: the sample hangs from a hook on a highly sensitive balance and is immersed in liquid, typically water. The force on the sample is recorded, and the contact angle is calculated from the liquid's surface tension and the sample's dimensions. Immersing the sample gives the advancing angle; withdrawing it gives the receding angle.
Because the result is averaged across the entire immersed surface, both sides of the sample need to be identical for the measurement to be meaningful. The method's sensitivity makes it well suited to very fine samples such as fibers.
Typical applications: fibers
Read more: Contact angle measurement of single fiber using Wilhelmy plate method
Washburn method
The Washburn method measures wettability through capillary rise: liquid is drawn up into a powder or other porous material, and the rate at which it advances into the packed bed tells you about the material's wettability. The value calculated from this is the advancing contact angle — a dynamic value, since it's derived from the speed of the liquid front rather than a drop at rest. The material is typically packed into a stainless-steel holder with holes in the bottom, which hangs from a force tensiometer and is lowered into liquid; the mass of liquid absorbed is recorded over time and used in the Washburn equation.
Before you can calculate a contact angle, you first need to establish the material constant using a completely wetting liquid. That constant depends on how densely and consistently the material is packed into the holder — which makes packing technique one of the most critical parts of getting a reliable measurement, since inconsistent packing is a known source of overestimated advancing angles.
Typical applications: powders, porous materials
Read more: Powder wettability
If you would like to learn more about contact angle and it's measurement techniques, please download the white paper below.
