Embodiments of this device or method repeatedly apply droplets of two or more liquids by means of nozzles of fixed relative direction in an alternate or sequential manner to a target location on a surface for removing material from the surface, adding material to the surface, or using the surface to biphasically catalyze a reaction of components of the liquids. The droplets have essentially no contact with one another before reaching the surface (FIG. 12A thru 13H). The effect of the droplets on the target surface can be modified by a continuous or interrupted flow of air or other gas to the target surface (FIG. 27A thru 29H), or by application of radiations such as sonic or ultrasonic radiation, or various frequencies of electromagnetic radiation, to the target surface, or some combination of these. Means may be included for adjusting the temperature of the liquids and gasses.