16
World Precision Instruments
If you assume the pipette tip is a cylindrical shape, the pressure of capillary action can
be described by the LaPlace equation:
P=4
γ
cos
θ
/
γ
= surface tension
θ
= contact angle between the water and glass
d
= inner diameter of the capillary where the meniscus is
located
n ost as s an assu
t onta t an or ass an at r s ro (un ss
t ass sur a s tr at ) ro t s uat on s t at t s a r t ns
diameter, the greater the capillary action. The capillary pressure can vary a thousand
times when the meniscus is moved from a 0.5 µm ID tip to the 0.5 mm shank.
r ssur at
t s a out PS ( n a u ous so ut on)
at t s an
be only 0.08 PSI. Using one regulator to counterbalance the pressure in such a large
dynamic range is not practical. 10 PSI can counterbalance a meniscus at the section of
tip where the inner diameter is 4 µm.
In practice, this is the highest pressure ever needed. On the lower pressure end, it
o
s
u t to a t ount r a an t a ar r ssur
n t
n s us
is at the shank of the pipette. However, a 0.1–0.2 PSI pressure imbalance will not
aus a s n ant ro
t t s s a nou
ra t o t u an t
o r s stan aus r t on ro t ass a
ot
to sto t so ut on
o at t s r ssur
TIP
t o st r ssur s tt n s n ff t at r
nt n t
tt u ro
leaking out, try setting the compensation pressure to zero to see if the gravity and
r t on ar su
nt to ount r a an t ront
TIP
a ar a t on an a so r u
a
n a
ro o u (su as
s on o )
n t
ro
sa n so ut on t an o
t
nat
s an n t s an o t
tt (s an at on n r as s t
θ
in the LaPlace equation
to
)
Manufacturing Micropipettes
Pulling suitable micropipettes is one of the biggest obstacles to taking full advantage
of the
PV850
ot ar an st a an s ar r u r
o u
o u
t s
markedly dependent on the micropipette tip size.
When using micron-sized tips a reduction in tip-size of a few percent may give an
or r o a n tu ff r n n t o rat t t s s ss t an
r ssur
t on
o
s n r as n
u t an s
a st s ust ta n
The most important of these steps is cleaning the glass. Small amounts of dust or
grease can easily clog micron-sized tips. Cleaning with chromic acid solutions before
pulling the electrode is commonly performed, but care must be taken to thoroughly
r ns t
tt s to r
o a tra s o t ro
a
as so
a n t or
glass. Some researchers prefer hydrochloric or nitric acid.
Silanization of the glass is also recommended for small tips. With 1 µm and smaller
tips, capillary action becomes prohibitively large, and the hydrophilic surface of the
