Chemistry Reference
In-Depth Information
Dispersants
Fillers (CaCO
3
, TiO
2
[whitener])
Alum
Defoamers
10.5.1.1 Inks and Printers (colloidal chemistry)
Over thousands of years for writing, the ancient people used naturally occurring col-
loidal fine material from ash (mostly charcoal) dispersed in oil (olive oil). Modern
inkjet printers employing color are based on much more sophisticated components.
Inkjet printers have a number of nozzles that inject ink droplets on the surface of
paper. Simultaneously, different colors are mixed to obtain the desired color shade
(more than hundreds of thousands). In a typical printer, there may be 30,000 injec-
tions per second, and there may be more than 500 nozzles (each with a size less than
a human hair (μm =10
−6
m). (The ink has a shelf life of more than a year.) In this
process, the surface and colloidal principles most obvious are
Contact angle (ink and the nozzle; ink and the surface of the paper)
Capillary pressure (at the nozzle)
It is thus seen that surface and colloidal principles are involved in a much complex
manner in these printers.
10.5.1.2 theory of adhesives and adhesion
Adhesives are used in everyday applications. Adhesives may be in liquid form or
thick pastes. Their main mechanism is based on the polymerization or crosslinking
of polymers, which gives rise to glue or other adhesive application. The degree of
adhesion of such a process is determined by conventional technological tests.
What follows from Young's equation is that, if a liquid is removed from the sur-
face of a solid, work will be needed, to execute this process (= Wad). This process
will require the destruction of 1 cm
2
of interface γSL, and the creation of 1 cm
2
of γ
S and γ L. From this we get
Wad = γ S + γ L − γSL
(10.1)
which, on combining with Young's equation, gives
Wad = γ L (1 + Cos (θ))
(10.2)
This shows that, to remove a liquid from a solid surface, surface tension and contact
angle θ are required. If the liquid wets the solid surfaces (water on glass θ = 0),
Wad = 2 γ L
(10.3)
Wad is thus the work needed to create twice the surface tension (one on each side
after separation or breakup).
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