Melissa Orme

EXPERIMENTS ON DROPLET COLLISIONS, BOUNCE, COALESCENCE AND DISRUPTION

There has been a significant effort to understand the events that occur when two or more droplets collide. The majority of the early work, which dates from the 1960s, focused on droplet growth relating to precipitation and was thus limited to water droplet collision phenomena. Recently, studies have sought information on the collision behavior of fuel droplets for application to combustion spray systems. Researchers have found distinct differences between the collision behavior of water droplets and fuel droplets. The extensive experimental data on the droplet collision process is reviewed and presented here for both water and fuel droplet collisions. Collision outcomes of bounce, temporary coalescence followed by separation or catastrophic fragmentation and permanent coalescence are examined and an effort is made to relate the existing findings to a unified description of collisional droplet behavior.

High precision solder droplet printing technology and the state-of-the-art

Abstract The continuing drive toward more complex integrated circuits (ICs) devices having lower cost, higher inputs/outputs (I/Os), greater operating speeds, increased functions per chip, and smaller device geometry has pushed the package requirements far beyond the capability of traditional packages, such as solder paste printing (SPP). Solder droplet printing technology is low cost, non-contact, flexible and data driven, and environmentally friendly has thus been discussed as an enabling technology for precisely placing fine solder deposits onto a variety of smaller substrates. It is therefore suitable for a variety of applications including direct chip attach site preparation, three-dimensional substrates, fine line interconnect, substrate via fill, optoelectronics and many others. It would also allow manufacturing techniques that are impossible or unfeasible with current technology, such as localized replacement of solder on board, depositing solder in different thickness on the same board, or using more than one type of solder on the same board. Recently, many experiments and studies have been under investigation by several groups. In this paper, the principle of the solder droplet printing technology is introduced and recent experimental results and potential application of the technology in the microelectronics industry are also included and evaluated.

Enhanced Aluminum Properties by Means of Precise Droplet Deposition

The use of molten aluminum droplets is investigated for potential application to precision droplet-based net-form manufacturing (PDM). In the proposed application, final structural components are made from the raw stock in one integrated operation by depositing molten metal droplets, layer after layer, via computer information. This work investigates the feasibility of the proposed technology by investigating the issues associated with generating molten aluminum droplets from capillary stream break-up, and examining the mechanical characteristics of the fabricated aluminum components. New results are presented which illustrate the generation of stable streams of molten aluminum droplets at rates of 24,000 droplets/second for a droplet stream speed of 10.9 m/s, corresponding to throughput rates of 2.3 × 10 -4 kg/s (1.85 lb./hour). The droplets travel 2,500 droplet diameters in an inert environment before impingement with the substrate. Microstructural images are completely devoid of splat boundaries, which have been removed by remelting, and the grain size is approximately uniform throughout the field of view of the image that, in most cases presented, contains easily upwards of 30 splats. Also, it has been found that the presence of aluminum oxide in the melt does not influence the average grain size of the component. An oxide barrier however will encapsulate each grain if the oxides are not removed by filtration in the pre-jetting stage. The presence of aluminum oxide in the melt does not prohibit the removal of the splat boundaries. Mechanical analysis shows that fabrication with molten aluminum droplet deposition results in a 30 percent increase in ultimate tensile strength compared to the raw ingot stock.

Electrostatic charging and deflection of nonconventional droplet streams formed from capillary stream breakup

Droplet streams generated from capillary stream breakup and forced with amplitude-modulated (a-m) disturbances will undergo a systematic interdroplet merging process due to their relative velocities that result from the a-m disturbance. This paper concerns the electrostatic charging of droplets generated with a-m 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 a-m 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 droplets).

Charged Molten Metal Droplet Deposition as a Direct Write Technology

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 electrostatic ally 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.

Cavitation of liquid streams in a vacuum

Conclusion In conclusion, a general formulation for diffuse configuration factors from planar point sources to disks is presented here. This formulation can be used to evaluate view factors for all possible positions of differential areas relative to disks. The present formulation can be reduced to the existing formulation by assigning numerical values to a number of its parameters. Finally, the resulting equations can be used to evaluate view factors from disks to finite surfaces.

The design and performance of a multi-stream droplet generator for the liquid droplet radiator

Results are presented for the performance capabilities of a multistream droplet generator suitable for use in a spacecraft liquid droplet radiator heat-rejection system. The nozzle-motion mode of stream perturbation initiation was tested with a single droplet stream and found to produce data similar to those generated with the resonant cavity mode of perturbation. Tests then proceeded to a 26-orifice array; the streams of the array responded to the perturbation satisfactorily, forming uniformly separated drops.