The measurement of tiny dew droplets at the initial deposition stage and dew point using a phase-shift interference microscope

The shape of a dew droplet deposited on the mirror surface of a copper plate was measured accurately using an interference microscope that employed a phase-shift technique. The microscope was constructed by adding a piezoelectric transducer to an interference microscope. A simple method that uses a conventional speaker horn and an optical fibre cable was used to depress any speckle noise. The shape of a dew droplet deposited at dew point on the plate surface with average roughness of 0.1 μm was measured with an accuracy of ±3 nm. The mass of a tiny dew droplet could be determined from the volume of its shape and was of the order of 10−9 g. The total mass of a dew droplet deposited per unit area and the deposition velocity under a gentle wind were also obtained in a similar way. The total mass was of the order of 10−5 g cm−2 at the beginning of deposition and the deposition velocity ranged from 1 × 10−5 to 6 × 10−5 g cm−2 min−1 at room temperature.