Hard Starch

1. Hard Starch Problem 13.

2. Introduction • Work devided in four steps • Achiveing effect • Necessary equipment construction • Measurement • Theoretical model development

3. What is corn starch ? Average particle radius is 3 *10-4 m It is not a corn flower !!

4. Achiveing effect “slowly”

5. Achiveing effect “fast”

6. Rotational viscometer • For rotational viscometer we have : r1 L r2 ω2 M Breznišćak mjerenje i računanje 1966.

7. motor Our rotational viscosimeter • It is different from standard • Cylinder is rotating • Current is linear to torque • Measurement accuracy 10-2A • Max. Velocity 0.628 m/s

8. Rotational viscometer .cont

9. Measurements at different concentrations

10. O d v a j a n j e Separation

11. Structure under microscope

12. Structure under microscope

13. Explanation • In state without stress – liquid phase • There is a water layer between particles • At low velocities water lubricates particles • Pressure increase – displacement of water between particles => direct contact • => Solution phase transition: liquid – quasisolid • Particles rub each other – Significant friction

14. Theoretical model • Model goals: • Estimation of Phase transition condition • Density • Pressure (streaming velocity) • Determination of drag dependence of velocity • Explanation and determination of effect for other solutions

15. 1. Transition conditions • Model geometry: • Layer structure is observed Surface 2 Surface 1

16. 1. Transition conditions • Parameter which determine contact is average distance between particles : • Contact condition: • We have to determine Γ hydrodinamicaly! Γ – coefficient N – number of particles per volume – Critical distance – Average particle radius

17. 1. Transition conditions cont. • Hydrodynamical contact condition : Separation of water boundary layer from particles • Criterion : Reynolds number • Separation at Re ~ 100 (In thin channel between particles) Π – geometry coefficient ρ – Liquid density (water) η – liquid viscosity v – Characteristic velocity (In our case upper surface velocity)

18. 1. Transition conditions cont. • Contact conditions combination: • Comparation with measurement: Rek – Critical Re number ρk – Critical density

19. 2. Drag dependence on velocity • Drag = Friction between particles + water viscosity • Linear to (Number of particles in contact)*(Force between particles) – Force adding! • N and F depends on pressure or (velocity)2 C – Coefficient Nef – Number of particles in contact Fij – Force between particles

20. 2. Drag dependence on velocity cont. • Dependence of N on pressure is estimated through asimptotyc behaviour: • P = 0 Nef =0 – every particle is surrounded with water • P Nef • Simplest function: N – Number of particles ξ - constant p – pressure

21. 2. Drag dependence on velocity cont. • Friction in solid phase – linear to pressure • => Dependence of drag on pressure : • Pressure becomes dynamical pressure • => Dependence of drag on velocity : Λ – linearity coeficient p – Pressure ρ – solution density v – average streaming velocity

22. Results comparation

23. Results comparation .cont Critical density