We demonstrate the possibility to achieve 3D particle focusing in a straight microchannel with a square cross-section by exploiting purely viscoelastic effects. Experiments are carried out by considering an elastic, constant-viscosity aqueous solution of PVP (polyvinylpyrrolidone) as the suspending liquid. Several flow rates and two channel dimensions (with a fixed particle size to channel dimension ratio) are investigated. A novel technique combining particle tracking measurements and numerical simulations is used to reconstruct the position of the flowing particles over the channel cross-section. The results show that, for all the investigated experimental conditions, particles migrate towards the channel centerline. Flow-focusing is enhanced by higher flow rates. The measured particle fractions can be rescaled according to a single dimensionless parameter, as already reported in the literature for the case of cylindrical channels. The so-obtained master curve can be used as a guide to predict the required focusing length. The effect of the entrance on the focusing channel length is also addressed. Finally, analogies and discrepancies with similar previous works are discussed.

Polymer solutions are considered dilute when polymer chains in a solution do not interact with each other. One important step in the characterization of these systems is the measurement of their longest relaxation times λ . For dilute polymer solutions in low-viscous solvents, this measurement can be very challenging through conventional techniques. Recently, several microfluidic platforms have been successfully employed to measure the rheological properties of weakly viscoelastic solutions. Nevertheless, a comparison between data generated from different microfluidic platforms has not yet been presented. In this work, we measure λ of dilute polymer solutions for concentrations down to a few parts per million, by using two distinct microfluidic platforms with shear and extensional flow configurations. We consider three representative polymer classes: Neutral polymers in near-theta and good solvents, and a biological polyelectrolyte in a good solvent in the presence of salt. Relaxation times in shear flow ...

We present a straightforward method for measuring the relative viscosity of fluidsviaa simple graphical analysis of the normalised position autocorrelation function of an optically trapped bead, without the need of embarking on laborious calculations. The advantages of the proposed microrheology method are evident when it is adopted for measurements of materials whose availability is limited, such as those involved in biological studies. The method has been validated by direct comparison with conventional bulk rheology methods and has been applied both to characterise synthetic linear polyelectrolytes solutions and to study biomedical samples.

A novel method to estimate the relaxation time of viscoelastic fluids, down to milliseconds, is here proposed.

In this work, we present new insights related to a debate on the morphological structure of hydroxyethyl cellulose (HEC) molecules when dissolved in water, i.e., whether HEC adopts a linear-flexible or a rod-like fibrillar configuration. We have employed “seven” rheological techniques to explore the viscoelastic properties of HEC solutions at different time and length scales. This work demonstrates an excellent convergence between various rheological techniques over a broad range of frequencies and concentrations, allowing us to derive microstructural information for aqueous HEC solutions without the use of complex optical imaging techniques. We find that when dissolved in water unmodified HEC behaves like a linear uncharged polymer, with an entangled mass concentration of c_e = 0.3 wt%. Moreover, for the first time we provide the concentration scaling laws (across c_e) for the longest relaxation time λ of HEC solutions, obtained from direct readings and not inferred from fitting procedures of fluids shear flow curves.