Optorheological thickening under the pulsed laser photocrosslinking of a polymer

Stephen Richard Okoniewski, Danielle Wisniewski, N. Laszlo Frazer, Weiqiang Mu, Andrew Arceo, Pranjali Rathi and J. B. Ketterson

Electro-, magneto-, and other rheological effects can be used to externally control fluid viscosity. However, they are largely reversible and in addition subject to colloidal settling, electrostatic breakdown, or high cost. In the experiments described here the dependence of the viscosity of a polymer solution under pulsed laser photocrosslinking as a function of radiation dose is determined using the Brownian motion of colloidal polystyrene tracers that were optically confined to a one dimensional channel. The system studied was a transparent aqueous solution of poly(ethylene glycol) dimethacrylate together with a 1-hydroxycyclohexyl phenyl ketone photoinitiator. An increase in the viscosity of the solution with the laser fluence was observed. The growth was exponential, stable between pulses, and spanned nearly three orders of magnitude.

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Investigations on rheological properties and gelation of tasar regenerated silk fibroin solution

Yogesha Lakkegowda, Raghu Ammannappa, Sharath Ananthamurthy
Tasar silk is a variety of non-mulberry silk indigenous to the Indian subcontinent. We present the measured frequency-dependent viscoelastic moduli of Tasar regenerated silk fibroin (RSF) solution using optical tweezers at two concentrations (0.16% and 0.25% w/v) and extend these measurements to the low frequency regime using a video microscopy technique. We extend the investigation on the rheological behavior of Tasar RSF for four more RSF concentrations, viz., 0.50%, 1.00%, 2.50% and 5.00% using video microscopy. In all the RSF samples, both storage and loss moduli are found to increase with frequency. At lower frequencies the loss modulus is more than the storage modulus and exhibit similar behavior until a crossover frequency beyond which the storage modulus exceeds the loss modulus at all frequencies. The relaxation time which is inversely related to the crossover frequency is found to rise sharply at 5% w/v, indicating the onset of gelation in the sample. These results are examined in relation to the viscoelastic parameters of mulberry silk, wherein the larger crossover frequencies at the same higher concentrations indicate relaxation times that are an order of magnitude smaller than those measured for Tasar RSF.

DOI