Rheometry-on-a-chip: measuring the relaxation time of a viscoelastic liquid through particle migration in microchannel flows

Abstract

A novel method to estimate the relaxation time of viscoelastic fluids, down to milliseconds, is here proposed. The adopted technique is based on the particle migration phenomenon occurring when the suspending viscoelastic fluid flows in microfluidic channels. The method is applied to measure the fluid relaxation times of two water-glycerol polymer solutions in an ample range of concentrations. A remarkable improvement in the accuracy of the measure of the relaxation time is found, as compared with experimental data obtained from shear or elongational experiments available in the literature. Good agreement with available theoretical predictions is also found. The proposed method is reliable, handy and does not need a calibration curve, opening an effective way to measure relaxation times of viscoelastic fluids otherwise not easily detectable by conventional techniques.

Is microrheometry affected by channel deformation?

“…1 Indeed, materials properties such as shear viscosity, 2 elongational viscosity, 3 and more recently relaxation times can all be obtained by means of microfluidic devices. [3][4][5] The main advantages of microrheometry reside in the very small amount of sample needed for the measurements (a very important issue for precious materials), and in the higher sensitivity possibly achievable with respect to conventional techniques, for some rheological properties at least. 5 In several instances, the measure of a characteristic relaxation time presents several difficulties.…”

“…[3][4][5] The main advantages of microrheometry reside in the very small amount of sample needed for the measurements (a very important issue for precious materials), and in the higher sensitivity possibly achievable with respect to conventional techniques, for some rheological properties at least. 5 In several instances, the measure of a characteristic relaxation time presents several difficulties. For example, fluids relaxation times down to milliseconds cannot be detected by conventional small angle oscillatory shear techniques simply because values of the storage modulus G 0 are extremely small ($10 À3 Pa) in the attainable frequency range.…”

“…For example, fluids relaxation times down to milliseconds cannot be detected by conventional small angle oscillatory shear techniques simply because values of the storage modulus G 0 are extremely small ($10 À3 Pa) in the attainable frequency range. 5,6 Recently, then, Zilz et al 4 proposed a microfluidic technique for the measure of the fluid relaxation time based on phenomena of flow-induced instability in a curved geometry, a serpentine microchannel in their work. This method, however, gives data with nonnegligible error bars due to the delicate nature of the flow instability.…”

“…This method, however, gives data with nonnegligible error bars due to the delicate nature of the flow instability. In our previous work, 5 we presented an alternative (and more accurate) way for measuring the longest fluid relaxation time in a microfluidic device, based on the viscoelasticity induced cross-flow migration of particles transversally to the main flow direction. 7 Particles suspended in a non-Newtonian matrix and subjected to a confined Poiseuille flow, indeed, migrate transversally to the flow direction 7 towards one or more equilibrium positions a) Present address: Micro/Bio/Nanofluidics Unit, Okinawa Institute of Science and Technology Graduate University, Okinawa, Japan.…”

“…While our previous analysis 5,9 has been carried out in Poly(methylmethacrylate) (PMMA) microchannels, most of the microrheometrical measurements available in literature have been carried out in Poly(dimethylsiloxane) (PDMS) channels, bonded on a glass coverslip. In fact, PDMS is the most used material for the fabrication of microfluidic devices.…”