Instruction manual

PV820/PV830 Pneumatic PicoPump
World Precision Instruments 13
NOTE: The pressure of capillary action is determined by the inner diameter of the
glass capillary where the meniscus of air/liquid interface is located. It has nothing
to do with the pipette tip size.
If you assume the pipette tip is a cylindrical shape, the pressure of capillary action can be
described by the LaPlace equation:
P=4
 cos/d
 = surface tension
 = contact angle between the water and glass
d = inner diameter of the capillary where the meniscus is located
In most cases, we can assume the contact angle for glass and water is zero (unless the
glass surface is treated). From this equation we see that the smaller the inside diameter,
the greater the capillary action. The capillary pressure can vary a thousand times when
the meniscus is moved from a 0.5μm ID tip to the 0.5mm shank.
The pressure at 0.5μm tip is about 80 PSI (in aqueous solution) while at the shank will
be only 0.08 PSI. Using one regulator to counterbalance the pressure in such a large
dynamic range is not practical. The hold regulator in this instrument is optimized to work
in the 0.2–10 PSI range. 10 PSI can counterbalance a meniscus at the section of tip
where the inner diameter is 4μm.
In practice, this is the highest pressure ever needed. On the lower pressure end, it
becomes diffi cult to exactly counterbalance the capillary pressure when the meniscus is
at the shank of the pipette. However, a 0.1–0.2 PSI pressure imbalance will not cause
a signifi cant problem if the tip is small enough. The gravity of the fl uid and the fl ow
resistance caused by friction from the glass wall will both help to stop the solution fl ow at
this pressure level.
TIP: If the lowest pressure setting is ineffective at preventing the pipette fl uid from
leaking out, try to switch off the hold pressure to see if the gravity and friction are
suffi cient to counterbalance the front fi ll. In addition, resetting the regulator could allow
the regulator to perform much better in this range (see the “Troubleshooting The Hold
Pressure” on page 19).
TIP: The capillary action can also be reduced by adding a hydrophobic fl uid (such as
silicone oil) behind the hydrophilic saline solution. It can be completely eliminated by
silanizing the shank of the pipette (silanization increases the  in the LaPlace equation to
90º).
Micropipette Manufacture
Pulling suitable micropipettes is one of the biggest obstacles to taking full advantage of
the PicoPump. Both care and steady hands are required.
The volume of fl uid ejected is markedly dependent on the micropipette tip size. When
using micron-sized tips a reduction in tip-size of a few percent may give an order of
magnitude difference in the fl ow rate. With tip sizes less than 1μm, pressure ejection
becomes increasingly diffi cult and special steps must be taken.