Nanoscale physics

Viscoelastic properties of starches measured by the ferrofluid droplet deformation method 
Azarkh D.1, Schnakenberg U.1
1 Institute of Materials in Electrical Engineering 1, RWTH Aachen, Aachen, 52074, Germany. 
E-mail: azarkh@iwe1.rwth-aachen.de
The viscoelastic properties of starches of different concentrations from normal corn (NC) and waxy corn (WC) were investigated. Ferrofluid droplets were injected into the starches by micropipettes. After applying constant magnetic stress, the time-dependent strain was calculated from measuring the minor and major radius of the elliptically deformed droplet. The obtained creep curves are shown in Fig 1. The creep behaviour of starch can be well described by an appropriate mechanical equivalent circuit, for example, the generalized Kelvin-Voigt model (GKV) or generalized Maxwell model (GM) [1]. While the GM model describes viscoelastic liquids, it would be better suited for a precise physical description of the experiment. However, GKV is easier to calculate and can be analytically described by following equation [1],

The extracted coefficients () from the upper fitted function can be transformed to the respective coefficients in GM () according to the known relations between the two models [1]. Values of elasticity and viscosity at two retardation time constants are presented in Fig. 1 A comparison with oscillatory tests [2] allows us to consider the delayed elasticity  and the viscosity  as the Young’s modulus and the viscosity of the starch.
	Fig.1 Creep curve of NC 7 g/100 mL (dots) and fitted function (line).  is the capillary stress from the ferrofluid droplet,  and  describe the instantaneous elasticity and viscosity, respectively, while  and  denote the delayed elasticity and viscosity
	
1.	Serra-Aguila et al. //J. Acta Mechanica Sinica 2019 -35.6 -1191-1209. 
2.	Villarreal et al. //Starch/Stärke  2016-68-1073-1083.