Page 60 - Physical chemistry understanding our chemical world

PROPERTIES OF GASES AND THE GAS LAWS 27

the ‘head’ is positioned above a part of the page to which an image is required, the
computer tells the head to eject a tiny bubble of ink. This jet of ink strikes the page
to leave an indelible image. We have printing.
The head is commonly about an inch wide, and consists of a row of hundreds of
tiny pores (or ‘capillaries’), each connecting the ink reservoir (the cartridge) and the
page. The signals from the computer are different for each pore, allowing different
parts of the page to receive ink at different times. By this method, images or letters
are formed by the printer.
The pores are the really clever bit of the head. Half-way along each pore is a
minute heater surrounded by a small pocket of air. In front of the heater is a small
bubble of ink, and behind it is the circuitry of the printer, ultimately connecting the
heater to the computer. One such capillary is shown schematically in Figure 1.8.
Just before the computer instructs the printer to eject a bubble of ink, the heater
is activated, causing the air pocket to increase in temperature T at quite a rapid
rate. The temperature increase causes the air to expand to a greater volume V .This
greater volume increases the pressure p within the air pocket. The enhanced air
pressure p is sufficient to eject the ink bubble from the pore and onto the page. This
pressure-activated ejection is similar to spitting.
This ejection of ink from a bubble-jet printer ingeniously utilizes the interconnect-
edness of pressure p, volume V and temperature T . Experiments with simple gases
show how p, T and V are related by the relation

pV
= constant (1.12)
T
which should remind us of both Boyle’s law and Charles’s law.

Ink jet nozzle

Resistor
t > 5 µs t ∼ 10 µs t ∼ 20 µs

Figure 1.8 Schematic diagram of a capillary (one of hundreds) within the printing ‘head’ of a
bubble-jet printer. The resistor heats a small portion of solution, which boils thereby increasing the
pressure. Bubbles form within 5 µs of resistance heating; after 10 µs the micro-bubbles coalesce
to force liquid from the aperture; and a bubble is ejected a further 10 µs later. The ejected bubble
impinges on the paper moments afterwards to form a written image. Reproduced by permission of
Avecia