The dark side of microrheology: Non-optical techniques

Abstract

Microrheology probes the mesoscale between bulk rheology, which provides an integral sample response, and nanorheology, which refers to a local response at a molecular confinement level. The term 'microrheology' is often used to refer to optical particle tracking methods that measure a local response of a sample. In contrast to this, non-optical microrheology techniques generally allow two different effects to be studied: actual confinement effects on the rheology and boundary effects such as slip. Investigating the mesoscale range has additional advantages such as the possibility to perform measurements with small sample volumes and at high shear rates. This review bundles the wide array of non-optical techniques into five categories: adaptations to a conventional rotational rheometer, sliding plate rheometry on a micrometer scale, microfluidics, piezo vibrators and radial channel flows. The concept of each set of techniques is described, together with their operational window and examples of recent studies.

Viscosity measurement based on the tapping-induced free vibration of sessile droplets using MEMS-based piezoresistive cantilevers

“…Conversely, for non-Newtonian liquids, the apparent viscosity depends on the applied shear rate 17,25,26,40,41 , which is proportional to the vibrational amplitude in the case of the proposed viscometer. For a Newtonian liquid, the viscosity of the liquid does not depend on the applied shear rate; therefore, the amplitude of the droplet vibration does not affect the results of the viscosity measurement, which allows the simple operating principle based on the tapping-induced vibration.…”

“…For example, measuring the viscosity of blood plasma is important in blood coagulation tests 1, 2 . In the recent development of microrheology, various techniques have been introduced to enable the measurement of liquid viscosity at the micrometre scale 17 . In these applications, a viscometer must be able to operate using a small sample volume because either only a limited volume of the target liquid is available or the target liquid is expensive.…”

Effective viscosity of a dilute homogeneous suspension of spheres in Poiseuille flow between parallel slip walls

“…From an experimental point of view, an appropriate set-up for measuring the effective viscosity of a dilute suspension would be the so-called microcapillary rheometer (see Vleminckx & Clasen (2014) and references therein). For comparing our predictions to experiment, the value of the wall slip length has to be known beforehand.…”

“…This is the domain of microrheology. In this field, non-optical microrheology is developing rapidly in search of technical solutions (Vleminckx & Clasen 2014). The so-called microcapillary rheometer is related to the problem considered here.…”

On the inseparability of slip and gap-error

“…5. The intercept will then be equal to the true shear rateγ b , and the slope will yield the value of the slip velocities V s or V δ [31,32,42]. Furthermore, in case of apparent slip, if the viscosity of the slip layer η δ = σ/γ b is known, the slip layer thickness δ can be calculated from the slip velocity V δ /δ =γ δ .…”