chemistry lectures presentation

1. Chemistry for textile
L06-07: viscosity and viscosity
measurement
Diffusion and laws of diffusion

2. viscosity
 It is the resistance
against flow of any
fluid.
 Viscosity of a solution
shows the degree of
entanglement of
molecules as they
move past each
other.

3. viscosity
 The viscosity of heavy
lubricating oil is
greater than gasoline
because of their
greater molecular
length.

4. viscosity
 Honey is much more
viscous than water
but if we dissolve a
gum in water, its
viscosity increases
too.

5. viscosity
 Why the viscosity of
water increases when
a gum is dissolved in
it?

6. viscosity
 Why some gum
solutions are thicker
than the others?

7. Viscometery
 Ostwald viscometer

8. Viscometery
 Obbelohde
viscometer

9. Viscometery
 Falling ball
viscometer

10. Viscometery
 Cup viscometer

11. Viscometery
 Rotary viscometer

12. Textile printing
 In textile printing
viscosity of printing
paste is very
important factor which
controls the quality of
pirinting.

13. 13
Diffusion
 Diffusion is defined
as the random
movement of
particles, in a
solution or gas, from
a region of higher
concentration to a
region of lower
concentration.

14. 14
Diffusion Flux
 Flux: the rate of migration of
matter is measured by it’s flux,
which is the amount of matter
passing through unit area per
unit time. Hence:
Flux = dQ/dt
Where dQ is a very small
amount of matter that passes
through a unit area in a very
small time period.

15. 15
Diffusion Gradient
 Concentration gradient:
Change in concentration of
solute while travelling a unit
length from the region of
higher concentration to the
region lower concentration
within a system. Hence:
concentration gradient = dC/dx
where dC is a very small
change in concentration over a
very small length in the
direction of decreasing
concentration.

16. 16
Diffusion
 The proportionality of the flux of matter to
the concentration gradient is called Fick’s
law of diffusion.
Fick’s first law is used in steady state
diffusion.
Fick’s second law is used in non-steady or
continually changing state diffusion.

17. 17
Ficks laws of diffusion
 Fick’s equations describe the diffusion of a dye within a fiber. Fick’s
second law sates that the rate at which the dye diffuses across a
unit area in the fiber is proportional to the concentration gradient
across that area.
dQ/dt is proportional to dC/dx
or dQ/dt = -D dC/dx
where:
Rate of diffusion = dQ/dt mol/m2/s
Concentration gradient = dC/dx mol/m3m
proportionality constant = -D (m2 s)
The negative sign gives a positive value of the diffusion coefficient
because the concentration gradient is negative as the concentration
decreases in the direction of diffusion (i.e. along the x axis)

18. 18
Diffusion – Importance in dyeing
 The transfer of a dye molecule from the dye solution into a fiber is
usually considered to involve the initial mass-transfer from bulk
solution to the fiber surface, that is adsorption of dye on the fiber
surface, followed by diffusion of the dye into the fiber. The diffusion
of dye within the fiber is thought to be the rate controlling step.
Diffusion in a polymer is much more difficult than in solution because
of dye fiber interaction and obstruction by the fiber molecules in the
pores. For example the rate of diffusion of direct dyes in cotton is
10000 times slower than in water.
 The concentration of adsorbed dye at the fiber surface quickly
reaches a steady state equilibrium value. Any net transfer of dye
from the solution to the interface then occurs only when the dye
diffuses into the fiber.

19. 19
Diffusion of dye within solution
 The diffusion of dye
within the solution is a
simpler process as
there is less
obstruction in the
process of diffusion.

20. 20
Diffusion of dye within the fiber
 The process of
diffusion into the fiber
is a slow step in
dyeing. The
complicated polymer
structure of fiber and
obstructions in the
pores control the
diffusion of dye
molecules within the
fiber.

21. 21
Diffusion of dye in dyeing system