You can measure the surface tension of ophthalmic solution by measuring the weight of drops of a fluid of interest falling from a capillary glass tube, and thereby calculate the surface tension of the fluid from this equation

mg=2*pi*r*σ

Where, weight of drops (mg) and radius of the tube (r) are known. (N.B.: You may search for Stalagmometric method)

Eur J Pharm Biopharm. 2004 Mar;57(2):377-82.

Evaluation of ophthalmic suspensions using surface tension.

Yasueda S1, Inada K, Matsuhisa K, Terayama H, Ohtori A.

Author information

1Senju Pharmaceutical Co. Ltd., Osaka, Japan. [email protected]

Abstract

Uniformity and precision of single dose are required for ophthalmic suspensions including water-insoluble ingredients. Solid sediments formed after standing still must be immediately re-dispersible and distributed homogeneously before use. However, selection of an appropriate water-soluble polymer as suspending agent is a challenging problem. In this report, the relationship between the surface tension and the re-dispersibility of suspensions was investigated. The surface tension of 0.1 w/v% fluorometholone suspensions began to decline from 74 mN/m at 0.0001 w/v% of hydroxypropylmethylcellulose (HPMC) and became almost constant at 52 mN/m at 0.01 w/v% of HPMC. Re-dispersion time was less than 4 s when HPMC was present at concentrations between 0.0001 w/v% and 0.01 w/v%. At these concentrations, aggregation of suspended particles was not observed. When indomethacin suspensions at 1.0 w/v% concentration were used, the surface tension began to decline from 73 mN/m at 0.0005 w/v% HPMC and became constant at 50 mN/m at 0.005 w/v% HPMC. The suspension also showed good re-dispersibility, and a uniform suspension was obtained between 0.0005 w/v% and 0.005 w/v% of HPMC. The time required for re-dispersion was less than 17 s. The change of surface tension showed a good correlation with the concentration of HPMC in ophthalmic suspensions having good re-dispersibility. Measurement of the surface tension of suspensions provided the optimal concentration of the water-soluble polymers for the suspensions of well re-dispersible characteristics. Evaluation of ophthalmic suspension using surface tension is a good strategy for formulation of suspending pharmaceutical products in the ophthalmic area.

From:

ELSEVIER

www.kibron.com/‎

www.kruss.de/‎

www.usp.org/sites/default/files/usp_pdf/.../ophthalmicpreparations.pdf

https://books.google.co.in/books?isbn=1566762138

https://ejournals.library.ualberta.ca/index.php/JPPS/article/.../5445