Biomedical Engineering Reference
In-Depth Information
100
First Peak
Second Peak
Linear Fit
10
1
1
10
100
k
(Pa.s
n
)
Fig. 7. Quality factor versus the consistency index
k
value for tomato puree and its dilutions
2.2 Viscoelastic materials
2.2.1 Semifluid materials
Squeezing flow is a well-known technique that has been applied to characterize the
properties of various biomaterials ranging from liquids to semisolids. The traditional
method involves measuring the force required to squeeze a sample between two cylindrical
disks either at a constant velocity or by applying a constant force or stress (Campanella and
Peleg 2002).
The oscillatory squeezing low method (OSF) uses the same geometry as the standard
squeezing flow method but it involves the application of small amplitude oscillations at
random frequencies up to 20 kHz (Mert and Campanella, 2008). The method allows one to
calculate both the viscous and elastic components of the sample viscoelasticity by measuring
the response of the material in terms of force and acceleration to those oscillations.
Transformation of the measured force and acceleration to the frequency domain yields a
frequency spectrum for the sample and, ultimately, its resonance frequency. From analysis
of this frequency response, two important viscoelastic properties of the samples, the loss
modulus G” (viscous) and the storage modulus G' (elastic), can be obtained.
The application of acoustic principles to the squeezing flow method is a novel technique,
which convert the squeezing flow method into OSF methos can measure the rheological
properties of materials that range from pure liquids to solids. Advantages of this method
include it being non-invasive, little to no sample preparation, and its ability to monitor rapid
changes during dynamic processes.
A schematic of the OSF testing apparatus is illustrated in Figure 8. The design uses a piezo-
electric crystal stack attached to an impedance head. Upon the application of voltage, the
upper plate oscillates, and the force and acceleration at the oscillating plate are measured
