Determination of contact angle of microspheres by microscopy methods

Confocal microscopy technique was used for contact angle measurement of individual microspheres. Glass microspheres with diameter of 3-10 micrometers were used for this investigation (this size range for particles are most relevant to inhalation applications). A contact angle of 41° ± 6° was calculated for these particles using different laser-scanned layers of the particle floating on the air-water interface when observed by confocal microscope. The ratio of the diameter for the cross section of the protruded area of the particle at the air-water interface to the actual diameter of the particle is used for contact angle measurements. The calculated contact angle of the glass particles was compared to that obtained from pendent drop method. For pendent drop, the apex of a water droplet with particles attached to it was observed from below. An inverted light microscope was used for observations. Contact angle can be measured using a simple geometrical relation and knowing the ratio of the diameter of cross section for protruded part of the particle from the droplet apex to the light colored ring around if (representing the particle diameter). A contact angle of 27° ± 5° was found for particles by this method. The plausible reasons for discrepancy between the two methodologies are discussed.