viscosity, adsorption, surface tension and osmosis

1. Course: BSc Biotech Sem-I
Subject: Biophysics and
Bioinstrumentation
Unit - 2
1

2.  The viscosity of a fluid is a measure of its
resistance to gradual deformation by shear
stress or tensile stress. For liquids, it
corresponds to the informal concept of
"thickness". For example, honey has a much
higher viscosity than water.
 The resistance experienced by one layer of a
liquid in moving over another layer is called
‘viscosity’.
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3.  Viscosity is due to the friction between
neighboring particles in a fluid that are moving at
different velocities. When the fluid is forced
through a tube, the fluid generally moves faster
near the axis and very slowly near the walls.
 A liquid's viscosity depends on the size and shape
of its particles and the attractions between the
particles.
 A fluid that has no resistance to shear stress is
known as an ideal fluid or inviscid fluid. Zero
viscosity is observed only at very low
temperatures, in superfluids. Otherwise all fluids
have positive viscosity.
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4.  Ether and gasoline have little viscosity and are
quite mobile.
 But honey and coal-tar have high viscosities.
 The unit of viscosity is the ‘Poise’ (1 P = 0.1
Pa·s), named after Poiseuille.
 It is expressed as,
41 2

5.  Temperature: The viscosity of liquids decreases
by about 2% for each degree rise in
temperature.
 Chemical composition: The viscosity of liquids
generally depends upon the size, shape and
chemical nature of their molecules. It is greater
with larger than with smaller molecules; with
elongated than with spherical molecules. Large
amounts of dissolved solids generally increase
viscosity. Small amounts of electrolytes lower
the viscosity of water slightly.
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6.  Colloid systems: The viscosity of lyophilic
colloid solution is generally relatively high.
 Suspended material: Suspended particles cause
an increase in the viscosity. The viscosity of
blood is important in relation to the resistance
offered to the heart in circulating the blood.
The heart muscle functions best while working
against a certain resistance. The viscosity of
blood is due largely to the emulsoid colloid
system present in plasma and to the great
proportion of suspended corpuscles.
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7.  Blood cells increase the viscosity of blood.
Higher the number of blood cells, the greater is
the viscosity.
 Blood viscosity helps in streamlining blood
flow by reducing turbulence.
 Blood viscosity helps in hemodynamics
especially in capillaries, blood flow through
narrow vessels.
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8.  The process of taking up substances from
solution on surface is called adsorption.
 Adsorption is the adhesion of atoms, ions, or
molecules from a gas, liquid, or dissolved solid
to a surface.
 This process creates a film of the adsorbate on
the surface of the adsorbent.
 The word "adsorption" was coined in 1881 by
German physicist Heinrich Kayser (1853-1940).
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9.  It is a surface phenomenon.
 The attractive forces on the surface are limited to
distances one molecule deep.
 The extent to which adsorption takes place is
dependent upon the nature of both adsorbing agent
and the substances absorbed.
 The greater the surface of the adsorbing agent, the
greater is the adsorption.
 Charcoal becomes activated when it is heated at 700-
800 C in a closed container and adsorption takes place
on the activated charcoal due to the attraction of
oppositely charged ions. Salts, acids and alkalis restrict
it.
 It has got much importance in industry.
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10.  Adsorption is a reversible process.
 It decreases with the rise in temperature.
 This process takes place relatively quickly.
Equilibrium is reached within one hour.
 Adsorption is proportional to the surface area
and it varies with the nature of the surface of
the adsorbent and of the substances to be
adsorbed.
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11.  It proceeds best from dilute solutions.
 Narrow pores on the surface of the adsorbing
agent are more effective than globular
openings.
 Heat is given off in all adsorption.
 The molecules adsorbed on the surface are
oriented and arranged in a definite manner.
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12.  Nature of the adsorbate
 Surface area of the adsorbent
 Effect of pressure
 Effect of temperature
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13.  Many chemical reactions are speeded up by the
presence of adsorptive surface.
 Surface adsorption helps to combine enzymes
with substrates to give reaction products.
 Adsorption processes taking place on the cell
membranes promote many vital chemical
reactions.
 Drugs and poisons which are adsorbed on cell
surfaces exert their effects from that location.
 The process of adsorption is applied in the
purification of enzyme.
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14.  The force with which the surface molecules are
held together is called surface tension.
 Surface tension is a contractive tendency of the
surface of a liquid that allows it to resist an
external force.
 Surface tension is an important property that
markedly influences the ecosystem.
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15.  The interior molecules of a
homogeneous liquid are equally
attracted in all directions by
surrounding molecules. They are free
to move in all directions.
 But the molecules in the surface of the
liquid are attracted downward and
sideways but not upward.
 As a result, the molecules of the
surface are not so free to move.
 They are held together and form a
membrane over the surface of the
liquid.
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3

16.  A great part of energy required to convert a
liquid into a gas is essential to overcome
surface tension and drag the molecules free
from the surface of liquid.
 Surface tension x surface area = Surface energy.
A falling drop of liquid assumes a spherical
form because the ratio of surface energy is the
least.
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17.  Surface tension = ½ hdgr
 Where, h=height of the liquid
d=density of the liquid
g=acceleration due to gravity
r=radius of the capillary tube
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18. 1. Substances that lower the surface tension
become concentrated at the interface.
2. Substances that increase surface tension tend to
move away from interface.
3. Lipids and proteins which are both effective in
lowering surface tension are found
concentrated on the cell wall.
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19. There are several methods of surface tension
measurements:
1. Capillary rise method
2. Stallagmometer
3. Wilhelmy plate or ring method
4. Maximum bulk pressure method
5. Hanging liquid drop or gas bubble method
6. Dynamic method
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20.  Surface tension decrease with increase in
temperature.
 Most inorganic salts slightly raise surface
tension of water.
 Organic substances usually lower surface
tension.
 Alkalis increase surface tension. Acid and
ammonia lowers it.
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21.  The presence of impurities either on the surface
or dissolved in it, affect surface tension of the
liquid. Highly soluble substances increase the
surface tension of water, whereas sparingly
soluble substances reduce the surface tension
of water
 the stronger the intermolecular attractive
forces, the higher the surface tension will be
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22.  Surface tension is involved in the process of
digestion; because bile salts reduce the surface
tension of lipids and thus assist emulsification.
As a result, surface area is increased which
favours lipase activity on lipids.
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23.  Osmosis is defined as spontaneous flow of
water from a more dilute to a more
concentrated solution when the two solutions
are separated from each other by a semi-
permeable membrane.
 Osmosis occurs in the direction opposite to that
in which diffusion occurs.
 Osmotic pressure rise with the rise in
temperature.
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24.  If the cell is kept in hypotonic solution, the cell
membrane will allow water to pass into it and will
set up an excess pressure in the interior of cell
causing the cytoplasm to be forced tightly against
cell wall. This condition is known as ‘turgor’ and
the cell is said to be turgid.
 If the cell is immersed in a concentrated solution
(high osmotic pressure), water will pass out of the
interior of the cell. The cytoplasm will then shrink
and detach itself from the cell wall. This
phenomenon is said to be ‘plasmolysis’.
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25.  Iso-osmotics: Solution with the same pressure
are termed iso-osmotics.
 Isotonic solutions: A pair of solutions which
produce no flow through a semipermeable
membrane are said to be isotonic solution.
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26.  Absorption from gastro-intestinal tract, as also
fluid interchange in various compartments of
body follows the principle of osmosis.
 Living red cells, if suspended in 0.92% NaCl
solution, neither gain nor lose water. Briefly
speaking, intracellular fluid of red cells is
isotonic with the red cell membrane in 0.92%
NaCl solution.
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27.  The osmotic pressure of a solution varies
directly with the concentration of the solute in
the solution and is equal to the pressure the
solute would exert if it would be a gas in the
volume occupied by the solution, if the volume
of the solute molecules relative to volume of
solvent be negligible.
 The osmotic pressure of a solution varies
directly with absolute temperature in the same
way as the pressure of a gas varies when its
volume is kept constant.
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28. Following methods are used for the
measurement of osmotic pressure,
(a) Pfeffer’s method,
(b) Morse and Frazer’s method,
(c) Berkeley and Hartley’s method,
(d) Townsend’s negative pressure method,
(e) De Vries plasmolytic method.
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29. READING IMAGES
 Text book of biochemistry
by West and Todd
 Biophysics and
Bioinstrumentation by N
Arumugam and V
Kumaresan
 Biophysical chemistry:
Principles and Techniques
by Upadhyay and Nath
 1-3:
https://www.boundles
s.com/chemistry/textb
ooks/boundless-
chemistry-
textbook/liquids-and-
solids-11/liquid-
properties-85/surface-
tension-376-6402/
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