An official website of the United States government
Electrospray deposition (ESD) has shown great promise for manufacturing micro- and nanostructured coatings at scale on versatile substrates with complex geometries. ESD exhibits a broad spectrum of morphologies depending upon the properties of spray fluids. Among them are nanowire forests or foams obtained via the in-air gelation of electrospray droplets formed from methylcellulose (MC) solutions. In this study, we explored MC ESD loaded with nanoparticles of various shapes and uncovered the effects of particle fillers on morphology evolution using coarse-grained simulations and physical experiments. Utilizing electrostatic dissipative particle dynamics, we modeled the electrohydrodynamic deformation of particle-laden MC droplets undergoing in-flight evaporation. The simulations quantitatively predict the suppression of droplet deformation as the size or concentration of spherical nanoparticles increases. While small particles can be readily encapsulated into the nanowire body, large particles can arrest nanowire formation. The model was extended to nanoparticles with complex topologies, showing MC nanowires emerging from particle edges and vertices due to curvature-enhanced electric stress. In all cases, strong agreements were found between simulation and experimental results. These results demonstrate the efficacy of the coarse-grained model in predicting the morphology evolution of electrosprayed droplets and lay the groundwork for employing MC nanowires for developing nanostructured composites.
more »
« less
Control of formation of viscoelastic droplets and distribution of nano-inclusions in functional deposition for lithium–sulfur batteries
The electrospray process produces micro/nanodroplets for various applications such as thin and uniform coatings, drug carriers and mass spectrometry. In this paper, we study the spray processes of viscoelastic jets using simulations and experiments. In discretized modeling, the jet is perturbed with axisymmetric instability and the growth of this instability causes the jet to break into droplets. For the experiments, a solution of polyvinyl alcohol in water is sprayed and is visualized using a high-speed camera. The droplet size distribution is studied from simulations with experiments for three spray cases: electrospray, air spray, and air-controlled electrospray. Our simulations and experiments reveal that the electric field is effective in reducing the droplet size, while air flow offers more jet break-ups and thus a larger number of droplets. As a result, air-controlled electrospray where these two driving forces are synergistically combined leads to a larger number of smaller droplets than electrospray or air spray. Finally, we applied three spray processes to obtain a deposition of sulfur/mesoporous carbon/graphene/polymer binder composites as a lithium sulfur battery cathode and demonstrated that air-controlled electrospray leads to a higher capacity and rate capability than other processes, exhibiting 800 mA h g −1 at 0.5C and 600 mA h g −1 at 2C.
more »
« less
- Award ID(s):
- 1719875
- PAR ID:
- 10147063
- Date Published:
- Journal Name:
- Soft Matter
- Volume:
- 15
- Issue:
- 32
- ISSN:
- 1744-683X
- Page Range / eLocation ID:
- 6485 to 6494
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Liquid droplet impact is a subject that has been investigated in both engineering and non-engineering applications to understand and to control this phenomenon. Spray cooling, ink-jet printing, spray coating and painting, soil erosion prevention, pesticide application, and impact erosion are merely a few examples in which droplet impact is involved. Erosion caused by droplet impact on a solid surface is important in numerous elements of industrial equipment, such as pipelines, steam turbines, and wind turbine blades. Though experimental and modeling studies have been performed on this topic, most failed to perform quantitative investigation especially when it came to the erosion of wind turbine blades. Moreover, most approaches assume that the impacting droplets are completely spherical and unaffected by any local turbulence or vortex shedding. As the droplet erosion process could be affected by several parameters, such as the impact velocity, shape and size of the droplets, this study focuses on investigating droplet properties and movement in a controlled lab environment. High speed imaging and Particle Image Velocimetry (PIV) methods are used for this purpose. PIV is used to measure the velocity, circularity, and size of the falling droplets in both disturbed and un-disturbed flow conditions. High-speed camera imaging provides additional insight to the path of the droplets’ movement in the presence of any turbulence. Experiments are performed at a variety of flow rates utilizing a range of blunt needle gauge sizes to create different droplet sizes. It is observed that the blunt needles produce a train of droplets that are different in size following each leading droplet. This is a crucial observation as it will have a direct impact on the magnitude of erosion and should be considered in the future modeling efforts.more » « less
-
We present experiments on large air cavities spanning a wide range of sizes relative to the Hinze scale $$d_{H}$$ , the scale at which turbulent stresses are balanced by surface tension, disintegrating in turbulence. For cavities with initial sizes $$d_0$$ much larger than $$d_{H}$$ (probing up to $$d_0/d_{H} = 8.3$$ ), the size distribution of bubbles smaller than $$d_{H}$$ follows $$N(d) \propto d^{-3/2}$$ , with $$d$$ the bubble diameter. The capillary instability of ligaments involved in the deformation of the large bubbles is shown visually to be responsible for the creation of the small bubbles. Turning to dynamical, three-dimensional measurements of individual break-up events, we describe the break-up child size distribution and the number of child bubbles formed as a function of $$d_0/d_{H}$$ . Then, to model the evolution of a population of bubbles produced by turbulent bubble break-up, we propose a population balance framework in which break-up involves two physical processes: an inertial deformation to the parent bubble that sets the size of large child bubbles, and a capillary instability that sets the size of small child bubbles. A Monte Carlo approach is used to construct the child size distribution, with simulated stochastic break-ups constrained by our experimental measurements and the understanding of the role of capillarity in small bubble production. This approach reproduces the experimental time evolution of the bubble size distribution during the disintegration of large air cavities in turbulence.more » « less
-
This paper presents a numerical model to describe the evaporation and drying of liquid droplets containing dissolved solids, relevant to processes like spray drying and spray pyrolysis in the food and pharmaceutical industries. A one-way coupled Euler–Lagrange approach is developed, where the gas flow inside the dryer is addressed in the Eulerian framework while the droplet dynamics and motion are tracked using the Lagrangian framework. In the Lagrangian framework, a novel and detailed kinetics-based multi-stage drying model is developed. The droplet level model is validated against experimental data for skim-milk droplet drying, showing strong agreement. Effects of different activation energy models are also analyzed and it is found that one model predicts the drying characteristics better than the other. Finally, large-scale three-dimensional simulations are performed on a lab scale spray dryer drying Maltodextrin solution in water. It is demonstrated that the model correctly predicts variation in final particle size distribution due to changes in drying air characteristics, paving way for the deployment of the model in predicting final particle characteristics in a spray dryer.more » « less
-
null (Ed.)Liquid-in-air generation of monodisperse, microscale droplets is an alternative to conventional liquid-in-liquid methods. Previous work has validated the use of a highly inertial gaseous continuous phase in the production of monodisperse droplets in the dripping regime using planar, flow-focusing, PDMS microchannels. The jetting flow regime, characteristic of small droplet size and high generation rates, is studied here in novel microfluidic geometries. The region associated with the jetting regime is characterized using the liquid Weber number (Wel) and the gas Reynolds number (Reg). We explore the effects of microchannel confinement on the development and subsequent breakup of the liquid jet as well as the physical interactions between the jet and continuous gaseous flow. Droplet breakup in the jetting regime is also studied numerically and the influence of different geometrical parameters is investigated. Numerical simulations of the jetting regime include axisymmetric cases where the jet diameter and length are studied. This work represents a vital investigation into the physics of droplet breakup in the jetting regime subject to a confined gaseous co-flow. By understanding the effects that different flow and geometry conditions have on the generation of droplets, the use of this system can be optimized for specific high-demand applications in the aerospace, material, and biological industries.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/c9sm01212e
