Drop shape analysis software such as CAST3.0 (USA KINO) or other one's is different when we used them to measure ultra-low interfacial tension of liquid-fluid system based on pendant drop method. CAST3.0 can fitting the drop shape more effectively while other one's may failure due to later adopts Young-Laplace equation fitting method based on select plane. (Hansen's method or B. Song's method). The fitting coefficient of Bond Number need to find experientially by a series of liquid that knowing Bond Number. But the lowest interfacial tension of knowing liquid is about 1.8mN/m, and drop shape of ultra-low liquid-fluid system is quite different.

optimization algorithm of CAST3.0 is ADSA or RealDrop method of 4th generation which introduced such algorithm as fitting and finding Bond Number at first fiting dimensional Young-Laplace equation process to find relevance between surface tension and radius of curvature, and then fitting Young-Laplace equation again to find surface tension. But on the contrast, Young-Laplace equation fitting method using select plane method calculated Bond Number by selecting some typical points (such as 30,45,60 crossing points or Ds/De planes). After Bond Number is calculating, they fit Young-Laplace again to find surface tension and radius of curvature.

Fig. 1:original pendant drop image of interfacial tension of liquid-fluid system. You should convert it to 8-bit BMP image format for drop shape analysis software made by other one's.

Fig. 2: Measurement of Interfacial tension of liquid-fluid system ( about 10^-1mN/m) using CAST3.0 based on RealDrop or ADSA method

Fig. 3: Measurement of Interfacial tension of liquid-fluid system using drop shape analysis software DSA1.9 by pendant drop method. We can find that fitting process is failure in this software.

Fig. 4: Measurement of Interfacial tension of liquid-fluid system using drop shape analysis software OCA20 by pendant drop method. We can find that fitting process is failure in this software.