Abstand

Selected physical properties


Viscosity

Types of rheological behaviour




Viscometers

are employed to measure viscosity.

The most simple instruments are

All these instrument should only be used with Newtonian or near-Newtonian liquids.

These two instruments can be used to record rheograms.

This instrument is more suitable for the control of batches of ink than for recording rheograms.




Viscosity ranges

for various classes of printing inks


The Hagen-Poiseuille law

says how fast liquids flow through (or penetrate into) a capillary:

So, a high viscosity of the liquid hampers its flow , but increasing the capillary diameter has a much more pronounced effect .
The pressure drop is brought about by the wetting of the inner surface of the capillary by the liquid. This will be investigated next.




Interfacial Tensions


Drops of water on a hydrophobic surface
Waterdrop on leaf
 
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Tenside")

Generally speaking, there are attracting forces between molecules of all kinds.

In the body of a homogenous liquid, these forces cancel out. At the surface of the liquid, the resulting force is directed inwards. The molecules at the surface attract each other, thus making the surface as small as possible. This is called the surface tension.

To produce extra surface, energy has to be expended; this is called the surface energy.


Splash
Waterdrop
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Tenside")



Note: The drops of water are spheres!

The dimension of a surface tension is force per length; in SI-units, this is N / m;
the dimension of surface energy is energy per area, [ N·m / m² ] = [ N / m ].
Obsolete, but still in use, are dynes / cm, which are part of the cgs-system.

Examples are (20 °C, unless stated)

Among others, surface tensions of liquids can be measured thus:

The term "surface tension" should only be used if one of the two phases is vapour or vacuum. The more comprehensive term is interfacial tension.

If, for example, a drop of a liquid sits on a solid surface, three interfacial tensions are to be considered:

These three quantities determine whether the liquid spreads over the solid, or not; whether it "wets" it.

This is judged by the contact angle, .

complete
wetting
incomplete wetting no
wetting


 = 0°  < 90°   = 90°  > 90°  = 180°

- > 0 - = 0 - < 0


Wetting (or not) of a solid surface by a liquid

Spreading stops, when the vectorial sum of the three foces is at right angles to the surface. The only degree of freedom the system has achieve this, is the contact angle, .



Wetting a solid surface
 [Full screen 67 KB]
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Tenside")


Interfacial tensions also bring about what is called capillarity.

If a small capillary is dipped into a liquid, the liquid usually rises within the capillary.The force causing this is multiplied by the distance it is operating, which is the circumference of the tube. Rising stops, when the weight of the liquid column equals the force upwards:

This holds for wetting liquids, which is usually the case. If wetting is incomplete, the equation has to be expanded: =>  · cos

for non-wetting liquids


Insect walking on the water

(Source: Fonds der Chemischen Industrie, Germany; imageseries "Tenside")



Often it is desirable to reduce the surface tension of water to increase its wetting ability. The most common way to achieve this is to mix it with another liquid, e.g. an alcohol, or to add a detergent, e.g. soap.

Such substances are called wetting agents. They reduce the attracting forces between the water molecules and hence reduce surface tension.

         

            


Drops of water on a textile surface
before and after addition of tensides
 
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Tenside")



Soap molecules, e.g. sodium stearate, consist of a long hydrocarbon "tail", which is lipophilic or oleophilic, and a small carboxylate "head", which is hydrophilic.

  H H H H H H H H H H H H H H H H H  O
  | | | | | | | | | | | | | | | | | //
H-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C
  | | | | | | | | | | | | | | | | |  \
  H H H H H H H H H H H H H H H H H  0-Na


Diagram of forces to explain the surface tension
 [Full screen 44 KB]
(Source: Fonds der Chemischen Industrie, Germany; imageseries "Tenside")



Hence, at a water-oil or water-air interface, all the soap molecules arrange in the same way: the head is immersed into the water, the tail points into the other direction.


(a) Grease on metal

(b) Greasy metal immersed in soap solution

(c) Grease surrounded by a film of
soap molecules forming a globule

(d) With agitation, the grease globule
separates from the metal


As a rule, the hydrophilicity of a substance increases as its lipophilicity decreases; in other words, hydrophilic substances usually are lipophobic, and lipophilic ones are hydrophobic.

Materials may also show both properties to an intermediate extent.

An exception from this rule are the silicones, which show neither of these properties.

                 




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