Sessile drop technique

The overall surface energy, both for a solid and a liquid, is assumed traditionally to simply be the sum of the components considered.

This is done by reversing the above idea with the introduction of a reference solid surface that is assumed to be incapable of polar interactions, such as polytetrafluoroethylene (PTFE).

A high contact angle indicates a low solid surface energy or chemical affinity.

For example, a contact angle of zero degrees will occur when the droplet has turned into a flat puddle; this is called complete wetting.

A droplet is deposited by a syringe which is positioned above the sample surface, and a high resolution camera captures the image from the profile or side view.

In order to measure the contact angle hysteresis, the sessile droplet can be increased gradually in volume.

[3] The advantage of this method, aside from its relatively straightforward nature, is the fact that with a large enough solid surface, multiple droplets can be deposited in various locations on the sample to determine heterogeneity.

Conversely, the disadvantage is that if the sample is only large enough for one droplet, then it will be difficult to determine heterogeneity, or consequently to assume homogeneity.

This is particularly true because conventional, commercially available goniometers do not swivel the camera/backlight set up relative to the stage, and thus can only show the contact angle at two points: the right and the left edge of the droplet.

The advantage of this method is that it is fairly objective and the measurement yields data which is inherently averaged over the wetted length.

Its disadvantages, aside from being more complicated than the goniometer method, include the fact that sample of an appropriate size must be produced with a uniform cross section in the submersion direction, and the wetted length must be measured with some precision.

In addition, this method is only appropriate if both sides of the sample are identical, otherwise the measured data will be a result of two completely different interactions.

However, the calculations described in the following sections, which were derived for the relation of the sessile drop contact angle to the surface energy, apply just as well.

The values obtained through the sessile drop technique depend not only on the solid sample in question, but equally on the properties of the probe liquid being used, as well as the particular theory relating the parameters mathematically to one another.

By constructing the Zisman plot, one can extrapolate the highest liquid surface energy, real or hypothetical, that would result in complete wetting of the sample with a contact angle of zero degrees.

This shortcoming is a result of the fact that the Zisman theory treats the surface energy as one single parameter, rather than accounting for the fact that, for example, polar interactions are much stronger than dispersive ones, and thus the degree to which one is happening versus the other greatly affects the necessary calculations.

To do this, one can simply reverse the procedure by testing the probe liquid against a standard reference solid that is not capable of polar interactions, such as PTFE.

The accuracy and precision of this method is supported largely by the confidence level of the results for appropriate liquid/solid combinations (as seen, for example, in fig 6[where?]).

First, one performs the standard sessile drop contact angle measurement for the solid in question and a liquid with a polar components of zero (

) The second step is to use a second probe liquid that has both a dispersive and a polar component to its surface energy, and then solve for the unknowns algebraically.

Though the principle equation is essentially identical to that of Owens and Wendt, the Fowkes theory in a larger sense has slightly different applications.

Because it is derived from different principles than Owens/Wendt, the rest of the information that Fowkes theory is concerned with is related to adhesion.

As such, it is more applicable to situations where adhesion occurs, and in general works better than does the Owens/Wendt theory when dealing with higher surface energies.

The van Oss theory[7] separates the surface energy of solids and liquids into three components.

It is naturally more robust than other theories, particularly in cases where there is a great imbalance between the acid and base components of the polar surface energy.

The most significant difficulty of applying the van Oss theory is the fact that there is not much of an agreement in regards to a set of reference solids that can be used to characterize the acid and base components of potential probe liquids.

The presence of surface active elements such as oxygen and sulfur will have a large impact on the measurements obtained with this technique.

For example, the presence of only 50 ppm sulphur in liquid iron will reduce the surface tension by approximately 20%.

In general, it is useful in determining the surface tension of liquids through the use of reference solids, with a similar technique being the Captive Bubble Method.

The two component theories would most likely be applicable to materials engineering questions about the practical interactions of liquids and solids.

An illustration of the sessile drop technique with a liquid droplet partially wetting a solid substrate. θ C is the contact angle, and γ SG , γ LG , γ SL represent the solid–gas, gas–liquid, and liquid–solid interfaces, respectively.
Sketch of the contact angle, as seen by a goniometer. In the top picture, the volume of the drop is being increased, and in the bottom it is being decreased. Each notated angle is an instance of a similar contact angle.