New Laser Technique for Automatic Interfacial Tension Measurements: Laser Scanning Drop Shape Analysis (LASDA)

Abstract

Laser scanners are widely used both in science and technology for precision measurements, e.g. in mechanical product control. Mostly they are used for the one-dimensional analysis of rigid bodies, allowing a precision of 0.2 μm. We have developed a new laser scanning apparatus which yields the coordinates of a pendant drop with an accuracy of 1−2 μm. The new tensiometer can be operated from a PC with special software that allows completely automated measurement and simultaneous calculation of interfacial tension. Using an interrupt technique, temperature control of the whole system and the mechanical control of two dispense systems are managed at the same time. The value of interfacial tension σ is obtained within 3 s (using a 60 MHz Pentium PC) by numerical integration (Implicit Euler Method) of the Bashforth Adams differential equations with about 100 drop coordinates. Fitting (Downhill Simplex Method) is carried out by variation of three parameters. The new tensiometer automatically yields the dynamic interfacial tension. The LASDA device includes two autocalibrating systems to achieve measurements without any external calibration. The laser scanner serves as a kind of ‘magic eye', controlling drop formation and break off as well as drop vibrations. Time-dependent changes are permanently shown on the measurement screen. The user software includes all features necessary for scientific and routine measurements; data administration and analysis, graphics, statistics, data export, and color printing. Due to special measuring programs the application range of LADSA is very wide: normal surface tension determination, dynamic measurements, relaxation studies, high-temperature/pressure measurements, etc. Examples of interfacial tension measurements are shown.