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Recipe for Sessile Drop IFT

Definitions

Sessile drops are sitting drops that rest on a substrate below or they are bubbles that rise up against a substrate from below. Pendant drops hang down from a tip and pendant bubbles rise up from a tip. Sessile drops are more difficult to analyze than pendant drops. The reason is the assumption in all drop shape measurements: the drop must be symmetric about its central vertical axis.

The pendant drop will normally be "large" with respect to its tip diameter whereas the sessile drop will be "large" with respect to its base diameter. The fact that the sessile drop has a large base diameter compared to its own diameter means that any fluctuations in the substrate affect the sessile drop's symmetry more than a pendant drop. Therefore the measurement tends to have more "noise."

We refer to the "apex" of the drop as the end of the drop away from the supporting solid structure. The apex of the normal pendant drop is at its bottom and the apex of the normal sessile drop is at its top. Sometimes the apex is called the vertex.

Substrate Choice and Setup

If possible, choose a substrate that has low surface energy, like Teflon. Low surface energy surfaces are not easily contaminated so they are more likely to be "clean" and chemically uniform.

Level the substrate so it is not tilting to one side. This is more important for IFT measurements than contact angles.

Drop Size

Sessile drop IFT measurements require relatively large drop volumes for good accuracy. This is contrary to contact angle measurements where small drops are often used. With contact angles, you do not want gravity to distort the drop. With IFT measurements you must have gravity distort the drop.

The relevant question is how "tall" the drop is. This height gives rise to a change in hydrostatic pressure between the bottom and top of the drop. This change in pressure changes the drop shape which is what we measure. As a rule of thumb, 2mm is a good height. With a pendant drop, this height can be obtained with relatively small volume, say 10ul, but with a sessile drop, perhaps 50ul will be required to reach 2mm of height. The height required depends on the interfacial tension of the liquid and its density. A distorted shape is what is required, irrespective of absolute height.

The required volume obviously depends on the contact angle of the drop against the supporting substrate. Higher contact angles lead to drops that "stand proud" and require less volume. This is a big difference from pendant drop analysis: pendant drop volume is essentially independent of the surface energy of the tip. Pendant drops really hang on the column of liquid above them, so they are not dependent on wetting the dispense tip.

Image Quality and Baselines

Sessile drop IFT measurements require good focus and a very clearly defined baseline. The consequences of a mistaken baseline are more severe for IFT than for contact angle measurements. Therefore you may wish to manually set a baseline slightly above the true liquid-solid interface to keep the measurement away from any local distortions at the three-phase line.

Reflection images, particularly when the contact angle is close to 90 degrees, also pose a challenge. The IFT measurement may not correctly separate the true image from the reflection on the surface. The extent of the measured region is indicated by the length of the vertical blue centerline in the image. If this goes below the true baseline, you will need to manually set a baseline by right mouse clicks then re-analyze.