Micro-sized Droplets Research Articles

CHALLENGES FOR FUTURE RESEARCH IN ATOMIZATION AND SPRAY TECHNOLOGY: ARTHUR LEFEBVRE MEMORIAL LECTURE

The current state-of-the-art of atomizer design, aircraft engines, diesel and gasoline engines, medical sprays, spray painting of automobiles, and laser optical instruments is reviewed. Future research for several decades will be driven by increasingly urgent demands to improve fuel and energy efficiency, and to drastically reduce the emission of pollutants. A much greater degree of control is required, not only in spray systems, but for pressure and flow rates which are introduced into atomizers. An improved understanding and control of liquid and air flows inside atomizers will be necessary. The breakup of liquid jets and sheets in many current spray systems results in chaotic generation of drop sizes and velocities. Electrosprays and ultrasonic sprays provide the means to generate more steady and controlled sprays. Major breakthroughs in the physics of diesel injector sprays have been achieved by using high intensity x-rays from the Synchroton at Argonne Labs. Moving away from single injectors to multiple injectors and replacing drilling of holes with etching and lamination allows multiple nozzles, with air and liquid swirl passages, to generate micro-sized droplets, fully dispersed in combustion chambers. The Federal Drug Administration has authorized the inhalation of insulin for treatment of diabetes. Aerosol particle sizers must be in the range of one to four microns. Stents, for the support of collapsed arteries, must be spray-coated with medication. Cryogenic sprays are used for cooling the skin during dermatological skin surgery. These are examples of important developments in the field of medicine. Future collaboration between physicists, mechanical, chemical and materials engineers, mathematicians, computationalists and experimentalists, with industrial designers and engineers, will offer many opportunities for young engineers to establish careers in atomization and spray technology.

Compromise between dominant polymerization mechanisms in preparation of polymer microspheres

A system consisting of uniform-sized styrene monomer droplets dispersed in an aqueous phase was employed to study the compromise between dominant polymerization mechanisms in the preparation of polymer microspheres. Monomer droplets were prepared by a microporous membrane emulsification technique, with negligible size distribution. Two polymerization mechanisms were identified—polymerization in micro-sized monomer droplets and nano-sized secondary nuclei, even in the presence of a hydrophobic initiator. Factors affecting the compromise between these two dominant mechanisms were investigated. Increasing the amount of hydrophobic additive (hexadecane) and/or adding water-soluble inhibitor in the aqueous phase suppressed the polymerization in the secondary nuclei, while adding a relatively hydrophilic monomer in the styrene feed and/or increasing the amount of the initiator and the size of the monomer droplets promoted the polymerization in the secondary nuclei. Under the condition where a hydrophilic monomer was added, hollow microspheres were formed when polymerization in the secondary nuclei was dominant, while one-hole microspheres resulted when polymerization occurred only inside the larger monomer droplets, indicating that the compromise between dominant mechanisms is critical to the polymerization process.

Zero-sized Effect of Nano-particles and Inverse Homogeneous Nucleation: PRINCIPLES OF FREEZING AND ANTIFREEZE

It was found that freezing of water in terms of homogeneous nucleation of ice never occurs even in ultra-clean micro-sized water droplets under normal conditions. More surprisingly, at sufficiently low supercoolings, foreign nano-particles exert no effect on the nucleation barrier of ice; it is as if they physically "vanished." This effect, called hereafter the "zero-sized" effect of foreign particles (or nucleators), leads to the entry of a so-called inverse homogeneous-like nucleation domain, in which nucleation is effectively suppressed. The freezing temperature of water corresponds to the transition temperature from the inverse homogeneous-like nucleation regime to foreign particle-mediated heterogeneous nucleation. The freezing temperature of water is mainly determined by (i) the surface roughness of nucleators at large supercoolings, (ii) the interaction and structural match between nucleating ice and the substrate, and (iii) the size of the effective surface of nucleators at low supercoolings. Our experiments showed that the temperature of -40 degrees C, commonly regarded as the temperature of homogeneous nucleation-mediated freezing, is actually the transition temperature from the inverse homogeneous-like nucleation regime to foreign particle-mediated heterogeneous nucleation in ultra-clean water. Taking advantage of inverse homogeneous-like nucleation, the interfacial tensions between water and ice in very pure water and antifreeze aqueous solutions were measured at a very high precision for the first time. The principles of freezing promotion and antifreeze and the selection for the biological ice nucleation and antifreeze proteins are obtained. The results provide completely new insights into freezing and antifreeze phenomena and bear generic implications for all crystallization systems.

Selective infiltration of superheated micro-sized droplets of a tin-lead alloy into a preheated powder layer for a novel solid freeform fabrication process

Selective infiltration of superheated micro-sized droplets of a tin-lead alloy into a preheated powder layer for a novel solid freeform fabrication process

Pulsed laser deposition of praseodymium-doped chalcogenide thin films

Praseodymium-doped GeS 2–Ga 2S 3–CsI thin films were prepared by the pulsed laser deposition (PLD) technique. We ablated chalcogenide glass, Pr 3+-doped, targets in vacuum with XeCl laser pulses (308 nm). The films were deposited on microscope slide glass substrates at room temperature. Film characteristics were investigated by many different techniques. The deposited films were well adhesive to their substrates. They were plain without cracks or corrugations and with stoichiometric composition close to that of the target. Micro-sized droplets were detected on the surface of films. The spectrophotometric measurements of transmittance and reflectance allowed to determine the real and imaginary parts of complex refractive index vs. wavelength, as well as the film thickness, by using a computer code. The waveguide guiding properties of the films deposited were investigated by prism-coupling technique (m-lines spectroscopy).

Formation and undercooling of droplets in the primary phase of Ag-Ge alloys

The formation of micro-sized droplets in the matrix of the primary phase of Ag–Ge alloys and their undercooling behaviour were investigated. Thermal cycling consisted in equilibration of hypoeutectic alloys near the eutectic temperature in the two-phase liquid–solid region followed by up-quenching of alloys at 30–50°C above the eutectic temperature and their subsequent slow cooling. A wide size range of droplets was generated in the matrix of the silver-rich phase by repeated thermal cycling of the alloy. During slow cooling, droplets of silver-rich liquid exhibited a maximum undercooling of 81°C below the eutectic temperature. Examination of the solidification structure of the undercooled alloys, by X-ray diffraction, polarized light microscopy and electron probe micro analysis techniques, indicated the presence of a metastable phase. The mechanism of formation of droplets and the nature of non-equilibrium solidification of the undercooled melt of silver–germanium alloys, are discussed.

A practical method for diameter, number density and material characterization of 40?90 �m size droplets by stimulated Raman scattering

We demonstrate that the technique of Stimulated Raman Scattering (SRS) is a practical method for the simultaneous characterization of diameter, number density and constituent material of microsize droplets. The method is applicable to all Raman active materials and to droplets with a diameter of at least 8 µm. Our experimental study was focused on water and ethanol monodisperse droplets in the diameter range of 40–90 µm. Results of a single laser pulse and multiple pulses are analyzed, indicating that the SRS method can diagnose droplets of mixed liquids and ensembles of polydisperse droplets.

Ice crystallization induced by silver iodide and bacteria in microsize droplets dispersed within emulsions

Abstract

A study by differential scanning calorimetry of the crystallization and the melting of D2O and D2O + H2O droplets dispersed within emulsions

Crystallization and melting of microsize droplets of D2O and H2O+D2O mixtures dispersed within an emulsifying medium have been studied by means of a Perkin-Elmer DSC 2 differential scanning calorimeter.

A study of the freezing of supercooled water dispersed within emulsions by differential scanning calorimetry

Supercooling of microsize droplets of water dispersed within emulsions has been studied by means of a differential scanning calorimeter Perkin-Elmer DSC 2. In an emulsion cooled steadily at 2.5K min-1, the breakdown of supercooling in individual droplets is distributed in temperature over the range -37.5 degrees C to -40.5 degrees C, the most probable temperature of freezing being T*=-(39.0+or-0.5) degrees C. When an emulsion is maintained at TC, 0 degrees C>TC>T*, the freezing of individual droplets is distributed in time, the time taken to achieve total freezing of the emulsion becoming shorter as TC becomes closer to T*. If an emulsion which has been partially frozen by being maintained at TC and then melted is cooled steadily at 2.5K min-1, then the distribution of freezing temperatures shows two most probable values of T*, -(34.0+or-0.5) degrees C and -(39.0+or-0.5) degrees C. This phenomenon is briefly discussed.