MEASUREMENT PROTOCOLS

 

 

 

VILASTIC SOFTWARE individually controls frequency, stress, strain, shear rate, time and temperature. By selecting a range over which measurements are made for one parameter while holding others constant, a variety of protocols can be generated to reveal the viscoelastic properties of a material.

Temperature Dependence: Water vs. A Polymer Solution

The figure shows automatic thermal scans of water and an aqueous polyacrylamide solution (Flopaam 2000 at 1000 ppm). Both cases were measured at a constant shear rate of 10/s and a frequency of 2 Hz. While the Flopaam shows both viscosity and elasticity, the water shows only a viscous component. Because of the high sensitivity the Vilastic instruments, fluids having the consistency of water can be accurately measured.

Strain Dependence of Viscoelasticity: Elastic Yield Stress

A xanthan gum solution at 2000 ppm in water was measured for a range of strains from 0.05 to 10 at a constant frequency of 2 Hz. Here, both the viscoelasticity and the shear stress are plotted versus the strain amplitude (rms). The stress is resolved into the elastic and viscous components. The peak in the elastic stress marks the point where the structure of the fluid is yielding to accommodate the stress. This point is the elastic yield stress.

Thixotropy and the Shear Rate Dependence of Viscoelasticity

Buttermilk was measured at a constant frequency of 2 Hz while increasing the shear rate from 0.1 to 70 / sec and then back to 0.1 / sec. The time for completion of the loop was 6 minutes. Both the viscosity and elasticity are larger during the initial, increasing shear rate. The microstructure lost during this phase is not recovered during subsequent measurements with decreasing shear rates. Thus the thixotropy affects both components of the viscoelasticity.

Time Dependence of Viscoelasticity

An example of gelation is shown in the formation of a blood plasma clot. Measurements were made every 15 sec for a period of 30 minutes at 2 Hz and a strain of 0.02. The transformation from a liquid to a solid is due to polymerization and cross linking of fibrin. This process is represented by the complex rigidity modulus of the material, the storage and loss moduli and the tangent of the angle of the modulus.

Frequency Dependence of Viscoelasticity

Hyaluronic acid at a concentration of 10 mg / ml was measured at a strain of 0.5 while stepping the frequency from 0.01 t o 10 Hz. The viscosity, elasticity, and relaxation time are shown. The viscoelasticity continues to change over three decades in frequency, showing that the relaxation spectrum is broad. The change of viscoelasticity with frequency can be measured while holding either shear rate, shear stress, or shear strain constant.

Measured and Computed Factors

29 factors are available for analysis, formatted for use in spreadsheet and plotting programs

  • Viscosity
  • Volume Flow
  • Elasticity
  • Time
  • Viscoelasticity Phase
  • Frequency
  • Viscoelasticity Magnitude
  • Radian Frequency
  • Loss Modulus
  • Max Energy Stored per Cycle
  • Storage Modulus
  • Energy Dissipated per Cycle
  • Tan (delta)
  • Q Factor
  • Rigidity Modulus Magnitude
  • Pressure (in phase with flow)
  • Viscous Stress
  • Pressure (in quadrature with flow)
  • Elastic Stress
  • Pressure Magnitude
  • Stress Magnitude
  • Volume Displacement
  • Resistance
  • Volume Flow
  • Reactance
  • Drive Level
  • Shear Strain
  • Temperature
  • Shear Rate

 

 

 

All details of the protocol are stored along with the 29 factors for each measurement. The storage file can be recalled at any time for further prints and plots of the results or for exact repetition of the measurements on other materials.

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