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Modeling the formation of electrospun fibers using a novel pedestal-shaped nozzle

Introduction

To fabricate and deposit micro and nanosized (polymer) fibers, a method called electrospinning is used. A typical electrospinning setup is depicted in Figure 1.

Figure 1: A schematic drawing of an electrospinning setup. The blue part in the syringe shows the electrospinning mixture

An electrospinning setup typically comprises of a capillary which is fluidically connected to an electrospinning mixture feed mechanism and is electrically connected to a high voltage (HV) power supply. The power supply changes the electrical potential of the capillary with respect to the grounded substrate, which is located approximately 10 cm below the capillary. If the power supply is switched off and an electrospinning mixture is pumped through the capillary (for example by means of a syringe pump), a pendant droplet forms at the end of the capillary. When the power supply is switched on, the local electric field at the end of the capillary changes the equilibrium shape of the droplet: a typical cone-shaped droplet is formed. This droplet shape is also referred to as Taylor cone [3]. If the (local) electric field is strong enough, the droplet cannot maintain an equilibrium shape and a fiber emerges at the apex of the Taylor cone. This thin fiber then traverses towards the substrate, where it is deposited [2].

Assignment

As depicted schematically in Figure 1, the electrospun fiber is first traversing in a straight path towards the substrate. However, the distance in which the fiber remains stable (i.e. it traverses in a straight path) is limited. There are many causes hypothesized which could cause the onset of the fiber instability. One of the most convincing theory is that the fiber surface suffers from small surface perturbations at the Taylor cone [1]. This perturbation grows very fast could cause the fiber to start whipping or, even worse, start to break up. In order prevent any induced surface perturbations of the electrospun fiber, the Taylor cone has to be kept as stable as possible. To do so, a novel nozzle-shaped nozzle is designed, fabricated and will be tested experimentally in the near future. These experiments will reveal data, such as but not limited to:

  • The size and stability (over time) of the Taylor cone;
  • the contact angle of the solid-liquid interface at the nozzle;
  • the voltage bandwidth and volume flow rate bandwidth in which electrospinning is possible;
  • the location (if applicable) at which the fiber instability occurs;
  • the influence of the nozzle dimensions to the above-mentioned variables.

We would like to analyze the experimental data and compare the data with simulations.

Using dimensional analysis, the experimentally obtained data will be nondimensionalized by the student. To do so, the student should think about appropriate non-dimensionless groups first. Then, using finite-element analysis programs such as COMSOL Multiphysics, the student will simulate the experimental situation. Lastly, the student will try to relate the simulation results to the experimental results and report the conclusions. We would like to stress that this assignment is only suitable for students who have experience with finite-element analysis programs, such as COMSOL Multiphysics.

More information 

Are you interested in this attractive project or do you have any questions about the project? Do not hesitate to contact either Prof. Dr. Han Gardeniers (j.g.e.gardeniers@utwente.nl) or Bjorn Borgelink (b.t.h.borgelink@utwente.nl).

REFERENCES

[1] M. Cloupeau and B. Prunet-Foch. “Electrohydrodynamic spraying functioning modes: a critical review”. In: Journal of Aerosol Science (1994).

[2] J. J. Feng. “The stretching of an electrified non-Newtonian jet: A model for electrospinning”. In: Physics of fluids 14.11 (2002), pp. 3912–3926.

[3] G. I. Taylor. “Disintegration of water drops in an electric field”. In: Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences (1964).