I have a hydrogel that visually looks like a gel (it is viscoelastic similar to mucus). When I do rheology on it, however it tends to show liquid-like behaviour (G''>G'), at higher angular frequencies (100rad/s), and sometimes solid-like behaviour at lower (1rad/s) frequencies.
I've used a 60mm plate and a 40mm plate (parallel) for these experiments. 200um gap height and 100um gap height, respectively. I do both strain (0.1% - >1000%, at 0.1, 1, 10 and 100 rad/s) and frequency sweeps (1% strain, 0.1->100rad/s).
Would anyone be able to advise why this may be? I'm not really sure how to further optimize my parameters as I've tried several different ones already.
Thanks!
The material may undergo shear induced breakdown of structure when subject to shear or may undergo slip in the rheometer. Try and see if you can eliminate slip. Shear induced breakdown of structure can simply be checked by loading the gel in a small bottle and shaking it vigorously.
From what you describe it seems that the material may not be a hydrogel. Are you sure that your material includes crosslinks? Have you performed a time sweep during the production of your material to see the G',G'' crossover (Winter definition of gel point)? In addition, the material may not have cured enough, meaning that the crosslinking reaction may has not finished.
Hi,
if you see such results consistently, the most probable answer is that your material is a viscoelastic liquid rather than a gel. It's practically impossible to distinguish it from a soft gel just by looking at it. You may decrease the temperature or the initial frequency. It's also possible that the material is shear thinning, and you disrupt it during loading - what is your loading procedure? Do you squeeze it by the upper geometry to 100-200 microns? That will cause a huge shear force, potentially damaging your sample's structure. Run a time sweep to see if the structure recovers or not. Slip is another issue, as mentioned by Balaji Iyer V.S. You may glue sandpaper to your geometry to get rid of it.
Thanks Petr Lepcio and Filippos F. Karageorgos
I'm confident it is a gel as it can be lifted with tweezers and can bear its own weight for a prolonged period of time; and when moved to water, it does not break down. It is, however, incredibly weak.
Gelation occurs almost immediately after mixing the two reagents, so while I have done time sweep experiments, using strains/frequencies at G'>G'' region of the frequency sweep curve, it shows solid like behaviour for the entire duration of the experiment (I don't get to see where it crosses over).
My main question is why at higher frequencies it shows liquid-like behaviour, yet at lower frequencies shows solid-like behaviour. My current understanding is that it may have to do with the %chains that are actually crosslinked vs chains that are are entangled, and how quickly they relax. Is this a plausible explanation?
Hi,
I am sorry; I misunderstood your question a bit. What you describe is a shear thinning, and it is the most common behavior in polymers at high shear. It's also common in other non-polymer gels, such as the physical gels based on interacting nanoparticles. The semi-empirical Cox-Merz rule says that you can interchange the complex viscosity from oscillatory measurements with the classical steady-flow viscosity at a frequency (expressed in rad/s, not Hz) with the same nominal value as the shear flow (expressed in 1/s). As far as I know, no one could explain why, and some complex systems do not adhere to this rule, but generally speaking, increasing frequency is equivalent to a higher shear rate.
Polymer chains introduced to shear are distorted in the direction of the applied field. At the same time, they recover their original steady-state shape through diffusion. At low shear, the recovery is faster than the diffusion, causing no visible effect on the viscosity. At higher shear, though, the recovery falls behind the shear distortion, getting the polymer chains aligned with the shear. This is a dynamic process, but you may consider a mean distortion getting increased at higher shears. If the shear disrupts weak interactions typical for soft gels, the gel liquifies. That's what you see in your data.
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