PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

Viscoelasticity measurement comparison between
microelastography and surface fluctuations
Pol GRASLAND-MONGRAIN1, Stefan CATHELINE2, Ali ZORGANI2, Damien GARCIA3,Ludovic BELLON1, Thomas GIBAUD1, Sébastien MANNEVILLE1
1

Laboratoire de Physique, Ecole Normale Supérieure de Lyon, Lyon, FRANCE
23LabTAU, INSERM u1032, University of Lyon, Lyon, FRANCE;

Creatis, Université Claude Bernard Lyon 1, University of Lyon, Lyon, FRANCE;

ABSTRACTMicroelastography is a recent technique used to measure viscoelasticity of small objects at the micrometerscale, such as biological cell. However, its results have not been compared to other viscoelasticity
measurements techniques at this scale.
We propose here to compare viscoelasticity results of microelastography with another based on surface
fluctuations. Results show a good agreement between both techniques over a large band of frequencies.
Keywords: Elastography, Shear Wave, Viscoelasticity

1. Introduction
Microelastography is a recent technique used to measure viscoelasticity of small objects at themicrometer scale, such as biological cell. It consists in inducing a shear wave propagation in an object
to map the object viscoelasticity. A recent work has applied the technique on a mouse oocyte, with amicropipette inducing a 15 kHz shear wave, and the oocyte was observed with a 200.000 fps camera
through a microscope. However, this technique lacks of gold standard due to the high frequency ofvibration.
In the other hand, another work has been able to measure sample viscoelasticity at a wide range offrequencies by observing at surface fluctuations.
The objective was to compare both techniques on the same viscoelastic medium.

2. Materials and methods
Both techniques have been tested on a gel sample. The gel was an acrylate sample (Gel coiffant,Casino, France) filled with 5 um microbeads (TS10 Dynoseeds, Microbeads, Norway) at 1%
concentration, and degassed during 10 minutes. Both experiments were done during the same day toavoid changes of viscoelasticity over time, with a similar environment temperature (20-22 °C).
For the microelastography experiment, to induce the shear wave in the sample, we used a 30 umdiameter pipette (MPH-MED-30, Origio, Denmark) with a piezoelectric actuator (PPA4-M, Cedrat
Technologies, France). The system was driven by function generator (HP33120, Agilent, USA) whichemitted a 1 V signal with 5 sinusoids of a given frequency, and a repetition frequency of 50 Hz . The
frequency varied from 100 Hz to 5000 Hz with a logarithmic spacing.
A high speed camera (v2512, Vision Research, NJ, USA) imaged the sample when vibration were
induced in the gel. Displacement was computed with an optical flow technique. Elasticityreconstructed with passive elastography.

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Figure 1. Microelastography technique illustration
Free surface thermal fluctuation setup was built using a 632 nm laser focused on the sample
surface. Reflected laser deviations were measured with a four quadrants photodiode. It uses a
viscoelastic model with a density ρ = 1000 kg, a surface tension γ = 130 mN/m, an initial elastic
modulus G (f = 0 Hz) = 100 Pa, a viscosity η = 0.02 Pa.s and a temperature T = 293 K. Surface
fluctuations among a band of frequency of 10 Hz to 100 kHz has been measured, giving the real andimaginary part of the viscoelastic modulus.

Figure 2. Free surface fluctuation technique illustration

3. Results and discussions
Microelastography provides 2D images of the elasticity, while free surface fluctuations measures
viscoelasticity only in one point.
The Figure 3 depicts the shear wave speed of the bulk measured by the microelastography at 500
Hz. Only the right part of the bulk was mapped, as shear waves didn’t reach the left side of thesample. The upper and lower left corner are due to a small optical artifact of the initial image. The
average value on the right side of the sample is 0.96 ± .05 m/s. With a sample density of 1000kg/m3 , this corresponds to an elasticity G* of 920 ± 100 Pa.

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Figure 3. Shear wave speed at 500 Hz as given by microelastography techniqueFree surface fluctuations and mean value of the microelastography technique results have beenplotted on Figure 4. Free surface fluctuations technique provides both the elastic and viscous part (G’and G’’), but for comparison with microelastography technique, only G* have been provided fromthese two values.
We can observe that although there is some difference above 1 kHz, both techniques measured thesame order of magnitude and depicted the same rheological behavior (increase of elasticity withfrequency).

Figure 4. Viscolelastic measurement of the gel sample along frequency measured by the freesurface fluctuation technique (red curve) and the microelastography technique (blue cross)We suppose the differences are due to viscoelasticity variation between surface and bulk of themedium. We plan to get measurements over a wider range of frequencies to get
Also note that the two techniques have different requirements, so cannot be applied
simultaneously for any materials: the first has to track shear waves, so a high viscosity may preventto measure anything far from the vibration source; the second needs a specular surface to reflect thelaser beam.

Acknowledgements
This work was supported by a postdoctoral fellowship award from the Fondation pour la Recherche
Médicale (PGM).

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