Mechanical vibrations of magnetically levitated viscoelastic droplets

Abstract

The mechanical vibrations of magnetically levitated droplets were investigated using a simple optical deflection technique. Droplets of water and a water-based solution of poly(acrylamide-co-acrylic acid ) were levitated in the bore of a superconducting magnet and perturbed with a short puff of air. Centre of mass and surface vibrations were monitored using laser light refracted through the droplet, focussed on to the end of an optical fiber and detected using a photodiode. Time dependent variations in the voltage generated by the photodiode were Fourier transformed to obtain the frequency and spectral width of the drops' mechanical resonances. A simple theory of drop vibration was developed to extract the rheological properties of the droplets from these quantities. The resulting values of G' and G" that were extracted were found to be in good agreement with values obtained using conventional rheology techniques.

Mechanical vibrations of pendant liquid droplets

“…Such effects show up predominantly in changes in the width of the spectra, although when damping effects become larger, some changes in the vibrational frequencies of drops can be observed [14]. Previous studies have considered the effects of viscous damping on droplet motion [1,19].…”

“…There has also been an interest in the use of droplet vibrations as a tool for measuring the material properties of liquids [7][8][9][10] and viscoelastic matea Corresponding author: james.sharp@nottingham.ac.uk rials [11][12][13][14]. These applications involve excitation of the vibrational modes of the droplets either by driving the droplets at their mechanical resonant frequencies [1][2][3]6] or by applying mechanical impulses to the drops [10,14].…”

Optovibrometry: tracking changes in the surface tension and viscosity of multicomponent droplets in real-time

“…However, the expressions derived here are modified slightly. This is caused by the fact that a vibrational node must exist in the contact region with the substrate if the droplets remain pinned during vibration-a condition which is not imposed on levitated drops [11]. This additional constraint has the effect of changing the prefactor in the relevant equations above when compared to the equivalent expressions for levitated drops.…”

“…Recent studies of viscoelastic fluids supported on superamphiphobic substrates [22] have also shown that their vibrational response can be described using similar models to those used for levitated drops of the same materials [11]. These authors showed that measurements of the drop size, vibrational frequency and width of the vibrational peaks can be used to extract the frequency dependent shear storage and loss moduli of high molecular weight poly(acrylamide-co-acrylic acid) solutions in water [22].…”

“…These authors showed that the vibrational frequencies and damping coefficients associated with the vibration of a free liquid globe (essentially a levitated liquid drop) could be related independently to the surface tension and viscosity of the fluid using a simple set of equations. More recent studies have shown that a similar approach can also be used to extract the frequency dependent rheological properties of viscoelastic fluids [11].…”