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Introduction

Surface and interfacial tension measurements can be performed optically using pendant drop shape analysis. The shape of the drop hanging from a needle is determined from the balance of forces which include the surface tension of the liquid being investigated.

The optical analysis of drops that hang from a dosing needle or are placed on a solid surface facilitates the determination of different surface and interfacial parameters. The contact angle that a liquid drop establishes on a solid surface characterizes the solid’s wetting behavior with said liquid.

Having measured the contact angles of multiple test liquids the surface energy of the solid can be determined and the latter can be used to calculate the work of adhesion for different liquids.

The reliable and experimentally robust measurement of the contact angle aids in the development of surface coatings, composite materials, paints, and varnishes or cleaning agents. In short: the measurement of contact angle helps in all situations where solids and liquids meet and an advantage is to be gained by the control of wetting and adhesion properties. 

Optical determination of surface and interfacial tension

When no other factor is in play a drop of liquid tends to form a sphere, due to its surface tension. The typical drop shape materializes because the drop is elongated due to gravity. The Young-Laplace evaluation of pendant drops recognizes this fact: The characteristic shape of the drop profile yields the surface tension σL of a liquid.

In the case where a pendant drop is surrounded by a second liquid, rather than air, the interfacial tension between the two liquids can be deduced from the drop shape. For optical analysis, the outer liquid has to be transparent. Depending on the relative densities, the inner liquid can be dosed either as a pendant drop or upwards, via a bent needle.

 

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