Microdroplet Generation Research Articles

On droplet formation from capillary waves on a vibrating surface

The formation of sprays from a liquid film on a vibrating surface is used by ultrasonic atomizers for applications ranging from humidification to metal–powder manufacturing. The received opinion in the literature is that droplets are formed periodically from the apexes of an orderly pattern of standing capillary waves, with a wavelength that can be related to vibration frequency by stability analysis. It is described how this assumption may be incorrect in that, after droplet formation commences, the orderliness of the standing–wave pattern is lost due to one or more secondary instability phenomena. These phenomena, which lead to disorderliness, are investigated by using high–speed imaging techniques and a low–frequency vibrating film to model the high–frequency case, because of the difficulty of penetrating clouds of small droplets in the latter case. Different modes of droplet formation are identified and the flow patterns responsible for these modes are discussed. Physical mechanisms are proposed from which it is deduced that only a proportion of standing–wave crests can eject droplets, for a given wall–vibration period, and the identity of these ejecting waves should vary from period to period. The model thus developed demonstrates an apparently random ejection of droplets from one wave cell, even though the model itself is deterministic. The disorder of the capillary waves and the occurrence of several droplet–formation routes are sufficient to explain the range of droplet sizes that is produced by ultrasonic atomization.

Spatial distribution of microdroplets generated in the cathode spots of vacuum arcs

Results of research on the spatial distribution of aerosol generated by the cathode spots of the vacuum-arc evaporators used for wear-resistant coatings deposition are presented in the paper. Research was carried out on four arc plasma sources differing in the cathode's structure and the arc ignition system. The spatial angular distribution for different fractions of microdroplets is presented. The influence of the cathode's shape on conditions of the microdroplets generation is described.

Transplantation of pancreatic islets contained in minimal volume microcapsules in diabetic high mammalians.

To minimize technical problems relating to excessive size (600-800 mu in diameter) of standard alginate microcapsules (CSM) for pancreatic islet graft immunoisolation, we have developed two novel minimal volume, chemically identical, capsule prototypes (MVC): 1) coherent microcapsules (CM), and 2) medium-size microcapsules (300-400 mu, MSM). CM, which envelop each individual islet within a thin alginate hydrogel cast, are prepared by emulsification, whereas MSM are made by atomizing the islet-alginate suspension through a special microdroplet generator. Upon graft into diabetic rodents, CM have shown to immunoprotect both allo- and xenogeneic nondiscordant islets, and restored normoglycemia. In higher mammals, at subtherapeutic doses, CM fully immunoprotected islet allografts (pig-->pig), but only temporarily xenografts (dog-->pig). We then used MSM to immunoisolate canine islet allografts in the peritoneal cavity of dogs with spontaneous insulin-dependent diabetes. Of three grafted dogs, two showed full remission of hyperglycemia with insulin withdrawal. MSM could represent an intermediate solution between CSM and CM for peritoneal immunoisolated islet transplants.

Study of microdroplet generation from vacuum arcs on graphite cathodes

The emission of microdroplets from the cathode surface in the vacuum arc ion plating deposition technique is the major drawback to the technique’s industrial use. The generation of these particles from graphite cathodes is studied in this article and correlated to the local thermal load in the cathode spot area. A pulse discharge was used for a precise control of this load. Increases in the arc current level, arc duration time, and, more generally, the local temperature of the cathode were found to increase the number and the average size of the emitted particles. Particles under these conditions also show an increase in the width of their size distributions. Increasing the distance between cathode and substrate was found to decrease the number density of particles observed on the substrate according to the solid angle covered. The microdroplets show a graphite structure and diameters between 0.2 and 2.0 μm. Conditions needed to decrease the number of particles emitted to the substrate are given.

Liquid microdroplet generator with glass orifice

A uniform size liquid microdroplet generator was fabricated with glass orifice. The orifice of the generator was made as small as 5 μm on a Pyrex tube. The size of the uniform ethanol droplets generated by the 5 μm orifice varied from 19 to 14 μm as the driving frequency changed from 150 to 400 kHz. Operation characteristics of this generator were comparable to those of the commercial devices. We also measured the interdroplet separation with the diffraction pattern of the array of droplets in order to get the velocity of droplet stream at various stream positions and driving frequencies. The deceleration of droplet stream was nearly independent of the driving frequency which determines the droplet size and interdroplet separation.