Mechanical strength of microspheres produced by drying of acoustically levitated suspension droplets

Floating synthesis with enhanced catalytic performance via acoustic levitation processing

Acoustic levitation supplies a containerless state to eliminate natural convection and heterogeneous crystal nucleation and thus provides a highly uniform and ultra clean condition in the confined levitating area. Herein, we attempt to make full use of these advantages to fabricate well dispersed metal nanoparticles. The gold nanoparticles, synthesized in an acoustically levitated droplet, exhibited a smaller size and improved catalytic performance in 4-nitrophenol reduction were synthesized in an acoustically levitated droplet. The sound field was simulated to understand the impact of acoustic levitation on gold nanoparticle growth with the aid of crystal growth theory. Chemical reducing reactions in the acoustic levitated space trend to occur in a better dispersed state because the sound field supplies continuous vibration energy. The bubble movement and the cavitation effect accelerate the nucleation, decrease the size, and the internal flow inside levitated droplet probably inhibit the particle fusion in the growth stage. These factors lead to a reduction in particle size compared with the normal wet chemical synthetic condition. The resultant higher surface area and more numerous active catalytic sites contribute to the improvement of the catalytic performance.

Modeling Drying Behavior of an Aqueous Chitosan Single Droplet Using the Reaction Engineering Approach

Spray drying of Chitosan solutions to prepare microparticles either using pilot or industrial scale spray dryer is a complex process; tracking morphological changes and obtaining drying kinetics of a single droplet would be very difficult. The acoustic levitator being a non-intrusive method is a useful experimental apparatus that enables particle/droplet suspension in the gaseous medium and capable of mimicking the drying process in a spray dryer. The drying of chitosan aqueous solutions into solid particles was investigated. The prediction of the size and drying kinetics until the formation of the solid structure was performed in an acoustic levitator. Studying the drying of single droplets is crucial for revealing the influence of the drying process parameters on the formation of dried particles. Droplets with initial chitosan concentration (10, 20, and 30 mg/ml) were investigated at different air-drying temperatures. A Reaction Engineering Approach (REA) model was developed and compared with the experimental drying curves, a very well agreement was found between the drying experiments and the REA model with a relative error of about 3% between the initial droplet mass and predicted droplet mass by the REA model.

Continuous micro-feeding of fine cohesive powders actuated by pulse inertia force and acoustic radiation force in ultrasonic standing wave field

Stable continuous micro-feeding of fine cohesive powders has recently gained importance in many fields. However, it remains a great challenge in practice because of the powder aggregate caused by interparticle cohesive forces in small capillaries. This paper describes a novel method of feeding fine cohesive powder actuated by a pulse inertia force and acoustic radiation force simultaneously in an ultrasonic standing wave field using a tapered glass nozzle. Nozzles with different outlet diameters are fabricated using glass via a heating process. A pulse inertia force is excited to drive powder movement to the outlet section of the nozzle in a consolidated columnar rod mode. An acoustic radiation force is generated to suspend the particles and make the rod break into large quantities of small agglomerates which impact each other randomly. So the aggregation phenomenon in the fluidization of cohesive powders can be eliminated. The suspended powder is discharged continuously from the nozzle orifice owing to the self-gravities and collisions between the inner particles. The micro-feeding rates can be controlled accurately and the minimum values for RespitoseSV003 and Granulac230 are 0.4 mg/s and 0.5 mg/s respectively. The relative standard deviations of all data points are below 0.12, which is considerably smaller than those of existing vibration feeders with small capillaries.