Measurement of interfacial dilational properties: A software-driven apparatus

The construction of a time-resolved surface viscoelastometer for the study of fluid/fluid interfaces is described in detail. Instrument design and operation are based on the formalism of systems theory. An interfacial-area change (i.e., an external stimulus) is generated according to a given time function; the ensuing dynamic surface tension response is continuously recorded in synchronism with the stimulus. The principal characteristic of the equipment is a computer-controlled mechanical motion of the area-confining device (i.e., of a particular ring barrier, in place of the traditional linear barriers). The software allows a wide variety of oscillatory or transient experiments to be conducted. Instrument performance is illustrated by a typical experimental response, obtained under conditions of small-amplitude area perturbation, for a submicellar aqueous solution of surfactant. The recorded trace is fitted to a theoretical two-parameter model. Results show the effectiveness of the controlled-surface-deformation technique, allowed by the instrument, for determining static and dynamic interfacial properties.