INTERFACIAL PROPERTIES AND PROCESS-STABILITY CRITERIA IN THE ELECTROSPINNING OF SILK FIBROIN SOLUTIONS: SURFACE TENSION, FIELD STRENGTH AND LOCAL FIELD ENHANCEMENT

Abstract

Electrospinning regimes are normally reported as an applied voltage and a working distance, and the surface tension of the spinning solution — the property those parameters must overcome — is usually quoted without an uncertainty estimate or omitted altogether. This work determines the surface tension of a silk fibroin / poly(vinyl alcohol) composite spinning solution by the stalagmometric method with a propagated uncertainty estimate, and relates it to the process regime through a physical analysis. The measured value is σ = 62.39 ± 1.22 mN m⁻¹ (relative uncertainty 1.96%), which is 14.8% below that of the reference liquid, a difference nine times the expanded uncertainty and therefore significant. Three distinct failure modes of the process were identified and traced to their physical causes: asymmetry of the Taylor cone, incomplete solvent evaporation in flight, and insufficient field strength to overcome surface tension. The stable regime corresponds to a mean field strength of 3.8 kV cm⁻¹, which is 3.4 times the mean of thirteen regimes reported in the tissue-engineering literature (1.12 ± 0.44 kV cm⁻¹) and is shown to follow from the exceptionally high viscosity of the solution. From the balance of capillary and electrostatic pressures, the local field at the Taylor-cone apex is estimated at approximately 2.4 × 10⁷ V m⁻¹, an enhancement factor of about 60 over the mean field, which quantifies the contribution of emitter-tip geometry to the critical condition. Because the local field scales as the square root of surface tension, the 14.8% reduction achieved by the ethanol component lowers the field required for jet initiation by 7.7%.