(405f) Controlling Surface Morphology and Spatial Distribution of Active Nanoinclusions in Functional Coatings Via Air-Controlled Electrospray Process

Authors: 
Divvela, M. J., Cornell University
Joo, Y. L., Cornell University
Electrospray process is used to produce electrically charged droplets of sizes in micron and submicron range. This process is used in diverse applications like production of thin and uniform coating, nanodroplet and nanostructure production, biomedical engineering, energy storage and mass spectrometry. In this process, the electrospraying liquid is pumped out of a needle through a syringe and an electric field is applied between the needle and the collecting plate. When the liquid is released out of the needle the electric field causes the charges to separate inside the liquid meniscus and it takes the shape of a cone at the needle, which is called as Taylor cone. When Taylor cone becomes unstable, depending on the process conditions either jets or droplets are ejected from the cone. In the spray process for viscoelastic liquids, the jet extends due to viscoelastic forces and the modulations on the jet radius due to axisymmetric instabilities cause the jet to break-up. Whereas, in non-viscoelastic liquids, the atomization of the meniscus at the nozzle is responsible for the formation of droplets. In the electrospray process, the electric field force is responsible for the growth of the axisymmetric instability and breaking the jet into droplets. In the air spray process, the external air flow is applied axially to the jet and the air drag accelerates the growth of the axisymmetric instability. In this work, we study the spray phenomena with simulations and experiments for the following cases: i) electrospray (only electric field), ii) air spray (only airflow) and iii) air controlled electrospray (airflow and electric field).

In this work, we use a Lagrangian discretized model as it is suitable to model both the jet breakup and droplet dynamics. In the discretized model, we consider the liquid jet to be made up of series of beads attached together with springs. The Newton’s second law of motion is applied to conserve the momentum. The jet at the nozzle is perturbed by introducing a normal mode disturbance to the radius of the bead at the nozzle. This disturbance results in axisymmetric instability on the jet and with the growth of the instability the jet breaks into droplets. We incorporate equations of motion for the formed droplets to study their dynamics. In this model, we also model the spray process due to external air flow by incorporating the air drag effects for air spray and air controlled electrospray cases.

In addition, we perform experiments to study the air controlled electrospray process of 8 wt% polyvinylidene fluoride in dimethylformamide solution. The air spray, electrospray, and air controlled electrospray processes are visualized with a high-speed camera. The average radius of the droplets obtained from experiments is compared with simulation results. The surface roughness and efficiency of the coating of PVDF/DMF solution on the collector are obtained to study the effect of air flowrate on air controlled electrospray of the solution. We also investigate the effect of air flow in air controlled electrospray process of 3% PVA/H2O solution with 15 vol.% Carbon Black or Carbon Nanotube particles to produce coatings with precisely controllable nanoscale topology, morphology, deposition thickness, and spatial distribution of active nano-inclusions. We further investigated the effect of additional deformation on the deposition homogeneity and the dispersion of carbon NPs (Carbon Black and Carbon Nanotubes) in resulting coatings, as it plays a crucial role in energy-storage applications.

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