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Normal force controlled rheology applied to agar gelation

Hydrogels of polymers such as gelatin or agarose (gelling agent obtained from red seaweeds and widely used in microbiology) are ubiquitous materials in the areas of agro-food and pharmaceutical industry. Composed primarily of water and a few percent in mass of polymers, these gels are obtained through the cooling of a polymer solution (sol - gel transition) which results in a weak sample contraction, traditionally considered as negligible. Depending on the extent of the gel shrinkage during the gelling process, the soft material confined in a cell of constant volume draws on the walls of the cell resulting in a strain hardening phénomenon [1] or a premature gel detachment from the walls. In a recent article in Journal of Rheology [2], CRPP researchers have shown that the weak contraction of agar gels has indeed a critical impact on both the measurement of elastic and viscous moduli of the soft material, and also on the monitoring of the gelling process. The cooling of the hot agar solution in a variable - volume cell subjected to a constant normal force equal to zero indeed allows more reliable and more robust measurements of the elastic properties of the forming gel as the soft material shrinks especially when the gelation is induced quickly. This method of measurement so-called "controlled normal force" is further used to show that both the elastic properties and the microstructure of agar gels obtained by the cooling of a hot polymer solution are independent of the thermal history. Agar gels elastic properties and microstructure neither depend on the cooling rate of the solution nor on an intermediate temperature step, which calls into question previous works and is of major industrial interest for the manufacture of gels in series, such as cell culture media. The "controlled normal force" measurement method extends beyond polymer gels and is also relevant for the determination of the mechanical properties of concentrated suspensions [3] and semi-crystalline solid fat materials like cocoa butter, chocolate or red lipstick for which solidification is also accompanied by a volume contraction of a few percent detrimental to the traditional analyses conducted at constant volume [4]. [5].

Article paru sur J. Rheol.

[1] https://en.wikipedia.org/wiki/Work_hardening

[2] B. Mao, T. Divoux and P. Snabre. J. Rheol. 60, 473-489 (2016)

[3] P. Mills and P. Snabre. Eur. Phys. J. E 30, 309-316 (2009)

[4] Kermarec A., Exsudation et rhéophysique des matériaux hétérogènes solide/liquide à base de corps gras. Thèse de doctorat de l’Université Bordeaux 1, octobre 2011.

[5] Habouzit D., Rhéophysique et stabilité des matériaux hétérogènes solide/liquide à base de beurre de cacao. Thèse de doctorat de l’Université Bordeaux 1, mai 2012.