Almost every electrospinning project meets the same early obstacle: SEM images showing fibers decorated with spindle-shaped beads, or a mat that looks more like sprayed droplets than fibers. Beads are not always a failure — some applications use them deliberately — but in most studies the goal is smooth, uniform fibers.

What causes beads

Beads form when surface tension wins over the forces that keep the jet continuous. Surface tension tries to minimise surface area by breaking the jet into spheres (a Rayleigh-type instability). Two things resist this: the viscoelasticity of the entangled polymer solution, and the electrostatic repulsion of charges on the jet surface.

So beads usually point to one of three conditions:

  • Insufficient chain entanglement — concentration or molecular weight is too low.
  • High surface tension — often from the solvent choice.
  • Low charge density — the solution conducts poorly, so the jet is not stretched strongly enough.

A step-by-step fix

  1. Increase polymer concentration in small steps. This is the most reliable remedy. Expect fiber diameter to increase as beads disappear.
  2. Use a higher-molecular-weight grade if raising concentration makes the solution too viscous to pump.
  3. Raise conductivity by adding a small amount of a soluble salt. This often both removes beads and reduces diameter.
  4. Adjust the solvent system, for example by blending in a co-solvent with lower surface tension.
  5. Re-tune flow rate and voltage so that the Taylor cone is stable. A cone that pulses or recedes into the needle often produces beads.
Diagnose before you change

Look at the cone while spinning. If it is stable but beads persist, the problem is almost always the solution. If the cone is unstable, start with flow rate and voltage.

When beads are useful

Bead-on-string structures can increase surface roughness and hydrophobicity, and beads can act as reservoirs for loaded compounds. If your application benefits from them, the same parameters let you produce beads in a controlled way.

For a broader view of how each variable shifts fiber morphology, read 7 parameters that control fiber diameter.