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Developments in LAOS for Microstructure Investigation, Part 2: Particle Arrangements

April 19, 2018

 

 

The previous blog article gave examples of LAOS data interpretations for non-covalent polymer gels.  Here the discussion is expanded to include how LAOS is being used to study the particle arrangements of hard particle colloids and suspensions.

 

Suspensions display a wide variety of mechanical behaviors under large strains that are relevant to mixing and storage applications. Polymethylmethacrylate nanoparticles were tested by Laurati et al. with LAOS to find the high strain required for particles to fully break down into individual particles or tightly connected clusters. (1) At strains just beyond the LVE, shear thickening colloids are believed to be “strain hardening”, meaning that they increase in stress with increased strain, possibly because of clusters becoming tightly held together in a gel-like network. (1) After more strain is applied, stress continues to increase, but shear thinning occurs after the structure starts to break down at an even greater applied oscillatory strain. Figure 1a shows a Lissajous plot example of the strain hardening with more rectangular curves over increased strain cycles. In the other Lissajous plot, the shear thinning in LAOS is identified through a more elliptical shape emerging over shear rate (Figure 1b). Strain hardening and shear thinning LAOS phenomena may occur for many colloids with polymer additives that induce attractive clustering, ultimately breaking down under large strains.

 

 

LAOS was also used to investigate the influence of poly(ethylene glycol) (PEG) in suspensions of laponite nanoplatelets, which are commonly added to coatings, adhesives, and consumer products to improve the ease of processing. (2)  Data comparisons between lower molecular weight PEG (4,000 and 10,000 g/mol) and higher molecular weight PEG (35,000 g/mol) revealed how longer chains may link laponite platelets while shorter PEG chains may induce depletion flocculation. (2) Evidence for this was seen in Lissajous plots, which displayed a more rectangular shape for the longer PEG chains whereas shorter PEG data was more elliptical. (2) Bridging effects of the long PEG chains resulted in a stiffer gel, while depletion gelation of the short chains formed a weak gel.  An example of a Lissajous plots for weak and strong gels is shown in Figure 2 from the previous post on LAOS for gels.

 

To differentiate between the microstructures of colloidal networks formed by polymer bridging flocculation, hydroclusters, and particle jamming, LAOS measurements were conducted by Khandavalli Rothstein. (3)  The particle-jamming cornstarch and hydocluster-forming fumed silica both displayed strain hardening in the Lissajous plots (example shown in Figure 1a). However, the polymer bridged flocs of hydrophilic fumed silica with polyethylene oxide chains displayed strain softening, shown by the development of elliptical curves over increased strain rate cycles (Figure 2). (3) This characteristic can be utilized to check the strength of inter-particle interactions and also to divulge the source of the gel strength. 

 

Many consumer products and foods undergo large strains while in use. Whey protein isolate (WPI) and κ-carrageenan gels, common food ingredients, were discovered to have different LAOS responses. (4)  LAOS data may be interpreted for manufacturing, food consumption, and potential correlation with specific food sensory attributes. (4)  Compared to WPI and mixed WPI-carrageenan gels, carrageenan networks displayed more strain hardening (Figure 1a) and required the least strain to deform. WPI gels and gels composed of both WPI and carrageenan were more resilient to large strains with elliptical Lissajous results, displaying their ability to recover from high strain. (4)  Such different responses mean that the carrageenan becomes more solid-like during high strain while being broken apart, whereas the whey protein structures disintegrate. This result revealed that WPI protein chain stretching may be responsible for the decreasing durability in the carrageenan gels.

 

Material structure has a direct impact on large strain mechanics. The more that is known about a sample’s behavior, the better it can be leveraged for product improvement. As more LAOS studies are conducted, the microstructural analyses will become more refined, gaining greater applicability to product development and formulation.  

 

Have questions about adapting rheometry methods for testing colloids? Contact us for a free 30 min. consultation.

 

 

References:

  1. Laurati, M., Egelhaaf, S. U., Petekidis, G., “Plastic rearrangements in colloidal gels investigated by LAOS and LSEcho”, Journal of Rheology, 2014, 58, 1395.

  2. Sun, W., et al., “Large amplitude oscillatory shear rheology for nonlinear viscoelasticity in hectorite suspensions containing poly(ethylene glycol)”, Polymer, 2011, 1402.

  3. Khandavalli, S., and Rothstein, J. P., “Large amplitude oscillatory shear rheology of three different shear-thickening particle dispersions”, Rheologica Acta, 2015, 54, 601.

  4. Melito, H. S., Daubert, C. R., and Foegeding, E. A., “Creep and Large-amplitude Oscillatory Shear Behavior of Whey Protein Isolate/κ-carrageenan Gels”, Applied Rheology, 2012, 22, 63691.

     

     

     

     

     

     

     

     

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