Microscopy and Morphological Fibre Identification

Fibre identification by microscopy (ISO 11087 and BISFA identification methods) uses longitudinal and cross-sectional fibre morphology at 100–400× magnification to definitively identify fibre type without chemical dissolution — essential as a first-stage screening method and for fibre types that cannot be distinguished by chemical means alone (e.g., cotton versus viscose, both dissolving in same chemicals). Longitudinal examination (transmitted light, 200× magnification): cotton — twisted ribbon form with characteristic convolutions (reversals 3–7 per mm), kidney-shaped cross-section; wool — cylindrical with overlapping scales 60–80 per mm length (scale height 0.3–0.5 µm, scale angle 15–25°, cuticle cell projection visualised at 400×); silk — smooth solid triangular filament, no fibre surface texture; viscose — fine longitudinal striations, serrated cross-section; polyester — featureless smooth cylinder (or trilobal if profile fibre); cotton versus linen: flax shows central lumen, cross-markings (nodes), and polygonal cross-section. Cross-section preparation: bundle embedding in wax or gelatin, microtome sections at 10–20 µm thickness on glass slide — cross-section essential for differentiating circular, trilobal, kidney, and serrated profile fibres. Polarised light microscopy: birefringence measurement (retardation in nm divided by fibre diameter in nm = birefringence Δn) — cotton Δn 0.046, flax 0.090, silk 0.059, polyester 0.175, nylon 0.060 — distinguishes crystalline natural cellulosic fibres from amorphous regenerated cellulosics (viscose Δn 0.010–0.020). SEM (scanning electron microscopy, 3–10 kV, gold sputter coat 5–10 nm) provides high-resolution surface characterisation at 500–10,000× — scale structure quantification on wool, surface treatment verification on carbon fibre, coating uniformity on functional fibres.