Microrheology with optical tweezers: measuring the relative viscosity of solutions 'at a glance'.

Manlio Tassieri,Tihana Bicanic,Pier Luca Maffettone,Neena Jain,Jonathan M Cooper,Francesco Greco,Emma J Robertson,Paolo Antonio Netti,Andrew Glidle,Francesco Del Giudice,Bettina Fries,Rab Wilson

doi:10.1038/srep08831

Abstract

We present a straightforward method for measuring the relative viscosity of fluids via a 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.

Highlights

We present a straightforward method for measuring the relative viscosity of fluids via a 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
With the advent of polymer physics, scientists established i) that, for very dilute polymer solutions, the viscosity increases above the solvent viscosity linearly with the polymer mass concentration, c, and ii) that the effective ‘virial expansion’ for relative viscosity is: gr 5 1 1 [g]c 1 kH[g]2c2 1..., where [g] is the so-called intrinsic viscosity and kH is the Huggins coefficient[6]

Summary

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Microrheology with Optical Tweezers: Measuring the relative viscosity of solutions ‘at a glance’. Nique only requires a few microlitres of sample volume per measurement and provides a straightforward and accurate procedure for measuring the relative viscosity of solutions – and the materials’s molecular weight via their intrinsic viscosity plus the Mark–Houwink law We demonstrate how this result can be achieved by means of a simple analysis of the normalised position autocorrelation function of an optically trapped bead, with the added advantage of avoiding laborious calculations that would involve either Laplace/inverse-Laplace or Fourier transformations of discrete time-dependent experimental data[26,27,28,29,30]. The advantages of the proposed method rely on its simplicity, and on its microrheology nature (i.e. it requires microlitres sample volume), which makes it of great interest to all those studies where rare and precious materials are involved, such as biomedical studies

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