Dynamic measurement of blood viscoelasticity by an oscillatory acoustic tweezing technique

Abstract

Acoustic tweezing rheometry is an innovative technology for low-volume non-contact rheological analysis of complex fluids characterized by increased sensitivity and accuracy as compared to traditional contact techniques. In this method, a small drop of a fluid sample is levitated in air by acoustic radiation forces and its viscoelasticity at different time instants is measured from drop shape changes. The acoustic tweezing rheometer operates in two different modes: quasi-static and oscillatory. This presentation focuses on the oscillatory technique in which the sample drop is forced into freely decaying shape oscillation by transient modulation of the standing acoustic field. Images of oscillating drops acquired by a high-speed camera are analyzed by a custom MATLAB code to obtain the shape amplitude vs. time curves and then the decay factor and resonance frequency of drop shape oscillation. Dynamic viscoelasticity of the sample (viscosity, relaxation time, and elastic modulus) was measured by applying the experimental data to the analytical formulae, derived from normal mode analysis of drop oscillation for viscoelastic fluid (Maxwell model) and viscoelastic solid (Kelvin-Voigt) materials. Using the oscillatory technique, we measured changes in blood viscoelasticity during coagulation and showed that the Kelvin-Voigt model leads to physically consistent results on rheology of coagulating blood.