Large amplitude oscillatory shear as a way to classify the complex fluids
Introduction
Recently there has been a growing interest in complex fluids, which include biological macromolecules, polyelectrolytes, surfactants, suspensions, emulsions, and so on [1]. These fluids are used in many fields of industry as food stuffs, personal care products, electronic and optical materials and for many biological applications. Complex fluids form complex microstructures depending on their thermal and deformation history conditions, which leads to diverse rheological properties. The rheological properties are usually investigated by two classes of flow. One is the linear viscoelastic experiment, as normally measured by the frequency-dependent storage modulus G′ and loss modulus G″, from which all the other linear viscoelastic properties can be calculated [2]. The other is a non-linear viscoelastic experiment which can be designed and performed in various ways. One example is the measurement of shear viscosity as a function of shear rate. The storage modulus and loss modulus reveal the mechanical properties of the material under small amplitude oscillatory shear, while the flow curve (non-linear behavior) provides the information at relatively large deformation. The measurement of G′ and G″ is often the most useful way of characterizing complex fluids. The moduli are usually predicted by molecular theories, and numerous experiments have been carried out to elucidate the microstructure of complex fluids.
Among the non-linear experiments, the step shear rate test (measurement of shear viscosity as a function of shear rate) is most often conducted. Since it provides the information regarding how readily the material can be processed or shaped into a useful product, it can be helpful in polymer processing operations such as injection molding. However, as the deformation is too fast at high shear rate, the material does not have enough time to respond to the environment and hardly provides any information about its microstructure. Another non-linear experiment is the large amplitude oscillatory shear (LAOS). It is normally generated by a strain sweep test, which is used to check the linear viscoelastic regime before the frequency sweep test. As storage modulus G′ and loss modulus G″ are defined only in the linear viscoelastic regime, the moduli at large strain lose their mathematical underpinnings, and have normally been neglected. However, provided that enough care is taken, the LAOS test can provide plentiful additional information. Furthermore, it allows both strain amplitude and time scale to be controlled independently, and it is easy to generate because it does not involve any sudden jump in speed or position [3].
In this paper, we will compare the rheological properties of some polymer solutions in both linear and non-linear regime. We will show the abundance of information on the microstructure in the non-linear regime, and the importance of LAOS experiment in revealing the interaction within the complex fluids as well as in characterizing the complex fluids.
Section snippets
Poly(vinyl alcohol)
Poly(vinyl alcohol) (PVA) is a unique synthetic polymer in that it has a large number of hydroxyl groups that can react with many kinds of functional groups. It has many applications, varying from thickening agent and dispersion stabilizer to solution-spun fiber. It is produced by saponification or hydrolysis of poly(vinyl esters), normally from poly(vinyl acetate). Its properties depend, among other things, on the degree of hydrolysis of the mother polymer [4]. The PVA sample used in this
Frequency sweep test
Fig. 4 shows the results of the frequency sweep experiment at 25 °C for each solution (PVA 16%, PVA 2%+Borax 1%, HA 1%+1 M NaCl, XG 4%). For all of the samples, except the XG solution, the loss modulus (G″) exceeds the storage modulus (G′) at low frequencies, indicating dominant viscous nature of the material under these conditions, while for XG solution G′ exceeds G″ indicating its dominant elastic nature. It is very important to control the viscoelastic properties of HA solution. The main
Conclusions
A non-linear dynamic test was found to be very useful in classifying complex fluids. Based on the large amplitude oscillatory shear (LAOS) behavior, we can classify complex fluids into four types: type I, strain thinning (G′, G″ decreasing); type II, strain hardening (G′, G″ increasing); type III, weak strain overshoot (G′ decreasing, G″ increasing followed by decreasing); type IV, strong strain overshoot (G′, G″ increasing followed by decreasing). The LAOS behavior of each class of fluid is
Acknowledgements
The authors acknowledge the support from the Korea Science and Engineering Foundation through the Applied Rheology Center at Korea University in Korea. The authors also acknowledge LGLS Ltd. for providing HA samples.
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