The drop shape analysis is an optical measurement method that can be used to directly or indirectly determine various properties of liquids and solids. For the measurement, an image of a drop is recorded, and the shape of the drop is evaluated.

Measuring Devices for Drop Shape Analysis

Using drop shape analysis, important surface and interfacial properties of liquids and solids can be determined. For the analysis of the drop shape, a drop is captured with a camera, and the contour of the drop is evaluated with software.

Essentially, a system for drop shape analysis consists of a camera, the sample to be analysed, and a light source. Optical contact angle meters, such as those in the OCA series from DataPhysics Instruments, can measure both contact angles and the static and dynamic interfacial and surface tensions. The drops are observed from the side by the camera, and the drop shape is evaluated with software.

With a spinning drop tensiometer, such as the SVT 25 from DataPhysics Instruments, particularly low interfacial tensions can be measured. In this case, the shape of a drop in a rotating capillary is analysed.

Methods for Drop Shape Analysis

With an optical contact angle meter, such as those in the OCA series from DataPhysics Instruments, various drop shape analysis methods can be performed. The so-called sessile drop method is used to measure static contact angles and to determine the surface energy of solids. In the sessile drop method, a droplet is placed on a smooth surface (Figure 2).

This method can be adapted to the captive bubble method. In this case, the surface of the solid is immersed in a surrounding liquid, and the droplet of the liquid to be measured or a gas bubble is dosed from below onto the immersed surface using a U-shaped needle.

The so-called pendant drop method yields the surface tension of liquids. Here, the droplet hangs from a needle tip (Figure 3). If this droplet is immersed in another liquid, the interfacial tension can be measured in this way.

In addition to the static values, dynamic measurement values such as the advancing and receding contact angles can also be determined from the evaluation. For this, further methods are necessary. The so-called needle-in method is similar to the sessile drop method: the droplet rests on a solid surface. Additionally, a needle is immersed in the droplet. It is used to dynamically enlarge and reduce the droplet, thus measuring the dynamic contact angles.

One method for measuring dynamic surface tension is the oscillating drop method. Here, the droplet is dosed hanging from a needle, as in the pendant drop method. At regular intervals, liquid is dosed into or withdrawn from the droplet, causing it to enlarge and shrink.

With a spinning drop tensiometer, such as the SVT 25 from DataPhysics Instruments, particularly low interfacial tensions can be measured. For this, the so-called spinning drop method is used. A capillary is filled with one liquid, and a droplet of another liquid is dosed into this liquid. The capillary is then rotated at high speeds, causing the droplet to deform depending on the rotational velocity.

Drop Shape Analysis: Evaluation

The drop shape analysis is performed using modern software that evaluates the recorded camera image. The contour of the droplet is detected using black-and-white contrast, and a mathematical equation is fitted to the shape of the droplet. Various models exist to describe the drop shape. Based on these models, values such as surface tension, contact angle, and interfacial tension are automatically output.