The versatility of the electro spinning technology allows controlling polymer fibre morphology and fibre deposition pattern, in order to fabricate engineered nano-fibrous porous membranes with a defined micro/nano-architecture, in terms of fiber size and fiber orientation. The membranes will be formed by entangled fiber with nanometric diameter, which will guarantee mechanical integrity joined to a very high specific surface area.
The development of new Electrospinning Lab Scale Prototype (ELSP) was realized as subcontract of LISSEN project founded by FP7 in tight collaboration with Pharmaceutical Department of University of Chieti – Italy. The new apparatus was required to realize an effective ‘scale-up’ of the electro spinning process of production of the S/C, Sn/C composites and polymeric fibers to obtain an increment of both the deposition speed and of the surface of the mats.
Preliminary Finite Element Simulation of classical single-nozzle geometry was used to set-up the necessary theoretical framework to be exploited. In electrospinning phenomena an electric field is applied to a flowing dielectric liquid. This is a multiphase, time dependent fluid flow problem where the applied electric field is coupled to the dielectric properties of the liquid solution. When a polymer solution is used also the charge density of the polymer should be considered. Considering the solution as “leaky dielectric” the charge density can be assumed to be zero in the bulk but there is a charge accumulation at the liquid-air interface because the presence of the electrostatic field. At the interface between air and solution, there is a coupling between the hydrodynamic forces of the two fluids and electro-dynamic charge accumulation at the interface. The shape of the interface between the two fluids is continuously changing under the action of forces resulting from different surface tension, density, viscosity and pressure (normal stress) that have to be balanced between the two fluids and accumulation of charges on the evolving liquid surface.
The Navier-Stokes equations describe the transport of mass and momentum for fluids of constant density. In order to account for capillary effects and liquid-air interface it is pivotal to include surface tension in the model. The Navier-Stokes equations are writing accordingly:
Where, is the density [Kg/m3], the dynamic viscosity [Ns/m2], u represents the velocity [m/s],
p is the pressure [Pa], I being the identity matrix and Fst the surface tension acting at the air/liquid interface.
The fluid dynamic and electrostatic problems are coupling through the Maxwell stress Tensor T. In particular is an electric force acting at the interface between the two fluid because the difference in relative permittivity of the medium. The force components are calculated in two dimensions as the divergence of the Maxwell stress Tensor:
The Finite Element simulations were able to predict the possibility to realize electro spun membrane from PEO water solution, substituting hazardous organic solvent. The more environmental friendly preparation of nanostructured fiber in water instead that in organic solvents is a relevant achievement of this development.
The design of the new 3D electrospinning apparatus the design of the new apparatus was obtain as the results of extensive numerical simulations. Increase production yield of nanostructured mat will require, not only optimizing the innovative geometry, but also predicting the behavior of different solutions to be spun when density, viscosity and conductivity are modified.
First simulation employing new geometry revealed that cylindrical bowl filled by polymeric solution could be used to create multiple jets contemporary. The electric voltage needed to seed the jet’s formation with this geometry increase proportionally with the radius of the bowl.
Figure 1 Simulation of jet formation of PEO in acetone
This is related to a reduced local charge accumulation at the solution/air interface, the charge accumulation occurs at sharp edges and vanishes as the radius of the bowl increases.
- Because this relevant but previously unexpected situation the scaling-up of the electro spinning process for the production of both the polymeric dry electro spun separators and composite electrodes will need a huge increase the electrostatic voltage exceeding the value of a standard laboratory apparatus. High electrostatic voltage (several hundreds of kV) poses also technical problem related to isolation and unwanted jet formation through surfaces of the experimental apparatus which are connected to ground like syringe pump dispensing the polymer solution. Also the quality of the fiber can be compromised at higher voltage value because multiple ionizations, which can create electrical current in the liquid, instability of the jets and formation of liquid droplets. Unwanted droplets decrease the yield and quality of the mat.
- In alternative a more complex geometry can be studied and realized for the laboratory prototype. The cylindrical symmetry of the bowl can be substitute with an apparatus with the shape of a star resembling a system with many needles distributed over a large cylindrical surface to avoid unwanted interactions and interferences between the jets. Possibly the active surface of the apparatus should rotate to create short jets, which will be squeezed by the movement of the surface, reducing charge accumulation and screening of the electric potential when large area of the mat is deposited on the opposite electrode.
Cylindrical design can be realized using circular internal emitter and a concentric collector. In this configuration the electric field required to start the jets was higher (35 Volt/mm).
The increase of the electric field (70 Volt/mm) with cylindrical configuration produces a growth of the jets lenght but also higher jets instability.
The next step was tought to modify the cylindrical geometry improving jet stability and jet separation. In the design will be introduced some small circular tooth along the cylinder surface. The idea is to guide the formation of the jets to reduce the instability with the hope to use smaller electric field taking advantage of the field accumulaation near the teeth on the surface.