Laser‐Assisted Contact Angle Measurements

ABSTRACT A laser beam, aimed at the solid‐liquid‐air interface of a sessile drop on a contact lens, produces two lines of diffraction which are theoretically normal to the profile of the fluid and solid surfaces at the point of contact. In the present experiments, the beam and the diffraction patterns were projected on a flat screen perpendicular to the beam, and the angle was measured as a contact angle. The angle was measured on curved surfaces (contact lenses) of 6.5 to 8.9 mm radius and for each material studied, the angle: (1) was independent of the front surface radius, (2) was independent of droplet volume at 2 and 10 μl, and (3) decreased with time after drop placement. The laser‐derived measurements correlated well with contact angles measured by goniometry on contact lenses and a variety of other materials (r = 0.86; p < 0.0001). Because the diffraction lines are straight and are generated at the point of contact, the angle formed by them is measured easily with a protractor, yielding a convenient and reproducible contact angle measurement. An application of the method was demonstrated by measuring contact angles for saline‐containing 0 to 2% bovine serum albumin or bovine submaxillary mucin on Silafocon‐A (Polycon II), Pasifocon C (Paragon EW), and polymethyl methacrylate (generic PMMA and Paragon 18) lenses. On each material, physiological concentrations of mucin, but not of albumin, significantly (p < 0.01) decreased contact angles by 8 to 15°. It is suggested that the laser‐assisted method is superior to goniometry because it is less subjective and is based on diffraction lines generated at the point of contact, rather than based on hand‐placed tangents to curved surfaces.