The average surface tension of the tears of six human subjects was found to be two-thirds that of water. The experimental method involved the use of a scleral contact lens which contained a circular tear collecting trough. Surface tension measurements using a fine wire ring were carried out while the contact lens rested on the subject's eyes.
Miller D. Measurement of the Surface Tension of Tears. Arch Ophthalmol. 1969;82(3):368-371. doi:10.1001/archopht.1969.00990020370014