Droplet streams generated from capillary stream breakup and forced with amplitude-modulated ͑a-m͒ disturbances will undergo a systematic interdroplet merging process due to their relative velocities that result from the am disturbance. This paper concerns the electrostatic charging of droplets generated with am disturbances, and seeks an understanding between the competitive effects of the electrostatic repulsive force and the momentum associated with relative velocities that are due to the am disturbance. To this end, experimental results and numerical simulations that are in excellent agreement and predict the configuration of the charged droplet stream are presented. In this work, droplets are generated with the aforementioned technique and are electrostatically charged. It has been found that for droplets generated with a disturbance with a high degree of modulation, m, the momentum of the droplets dominates over the electrostatic force causing the droplet merging process to be similar to the uncharged case as attested by both experimental results and numerical simulations. Here, m is the ratio of modulation to original carrier amplitudes. For droplets with lower values of m, the electrostatic forces dominate and it is found that the relative positions of the droplets will oscillate along the direction of their flight path with maximum excursions not exceeding one wavelength of the carrier disturbance ͑i.e., the separation of the unmerged droplets͒.

The formation of highly uniform charged molten metal droplets from capillary stream breakup has recently attracted significant industrial and academic interest for applications requiring high-speed and high-precision deposition of molten metal droplets such as direct write technologies. Exploitation of the high droplet production rates intrinsic to the phenomenon of capillary stream break-up and the unparalleled uniformity of droplet sizes and speeds attained with proper applied forcing to the capillary stream make many new applications related to the manufacture of electronic packages, circuit board printing and rapid prototyping of structural components feasible. Recent research results have increased the stream stability with novel acoustic excitation methods and enable ultra-precise charged droplet deflection. Unlike other modes of droplet generation such as Drop-on-Demand, droplets can be generated at rates typically on the order of 10,000 to 20,000 droplets per second (depending on droplet diameter and stream speed) and can be electrostatically charged and deflected onto a substrate with a measured accuracy of ±12.5 µm. Droplets are charged on a drop-to-drop basis, enabling the direct writing of fine details at high speed. New results are presented in which fine detailed patterns are “printed” with individual molten metal solder balls, and issues relevant to the attainment of high quality printed artifacts are investigated.