Captive bubble measurements

Measuring contact angles on highly wetting or absorptive surfaces is problematic because water droplets may not reach a measurable equilibrium value before disappearing or going to 0°. One may also want to characterize a material that has underwater or hydrated applications in its use case environment such as with contact lenses. Because standard contact angle measurements are not feasible in these two situations, an alternative measurement one can make is a captive bubble measurement. In this setup, the sample is hydrated in liquid and a hooked needle is used to deliver an inverted air bubble or immiscible liquid onto the sample.

Figure 1 – Accessories such as a hooked needle (C210-22), cuvette (C208A), and threaded syringe (C205) are useful for performing captive bubble measurements.

To prepare a sample for captive bubble measurements, one can place the sample in or on top of a liquid vessel. If the sample is placed inside a vessel, an appropriate device must be used to immobilize it. If it is placed on top of a liquid vessel, the vessel must be fully filled with liquid to ensure that the meniscus hydrates the sample sufficiently. Care should be taken to ensure that the sample remains as flat as possible and is not wrinkled or warped during the setup, as doing so can distort the baseline. Then, an air pocket is taken up by the syringe so that an air bubble can be placed on the sample. The hooked needle is positioned so that the outlet is just below the sample surface, and the syringe head is twisted until an air bubble releases from the needle onto the sample. Advancing and receding measurements can also be performed by placing the needle inside the bubble and increasing and decreasing its volume.

Data analysis is performed similarly to standard contact angle measurements, with the main difference being that the “Flip Y” option must be checked so the software recognizes an inverted shape instead of a standard one. High contact angles indicate a high affinity of the surface for the liquid media, and vice versa. Because the captive bubble measurement inverts the air-liquid interface [1], the bubble contact angle relates to the liquid measurement with the equation θ_L=180^∘-θ_B, where θ_L is the liquid contact angle and θ_B is the bubble contact angle. For example, in Figure 2, an equivalent liquid contact angle is roughly θ_L = 30°.

Figure 2 – An example of a captive bubble measurement. The “Flip Y” function, which is necessary to perform analysis on inverted contact angle measurements, is highlighted.

Conclusions

Captive bubble measurements are an alternative way to make measurements on absorptive, hydrated, or porous samples. A hooked needle filled with air is placed below the sample and a bubble is dispensed onto the surface. Analysis of the bubble contact angle is performed in the same way as a traditional measurement with liquid drops, with the exception that the baseline is above the drop instead of below it. The contact angle obtained can be directly related to the water contact angle with the equation θ_L=180^∘-θ_B.