β-Lactoglobulin films with spiral cracks

We studied spiral crack formation in dried β-lactoglobulin films. The spirals result from simultaneous crack propagation and fragment detachment and the final pattern can be mathematically described by a logarithmic spiral equation. Crack patterns were investigated under different β-lactoglobulin and plasticizer concentrations of the initial solution used for film preparation. Spiral crack patterns occur in films over the entire tested concentration range up to cm = 250 mg/mL, but disappear when more than 3% glycerol or sorbitol are added as experiments at a fixed concentration of β-lactoglobulin (cm = 100 mg/mL) show. While the estimated outer size of the spiral increased steeper than the inner one with protein concentration, an equal increase for both values was observed in dependence of the plasticizer concentration. The corresponding k-values obtained from logarithmic spiral fits show a strong dependence on both protein and plasticizer concentration. By a simple model based on the speeds of the fragment detachment and the crack tip we show why two spiral types form in protein-concentration dependent experiments but only one when the amount of plasticizers is varied.