Measuring Surface Tension: Part 2
Barbara Kanegsberg & Ed Kanegsberg, September 2003
In Part I of Measuring Surface Tension, we introduced the concept of measurement
of surface tension of a liquid as an important parameter for many applications.
We will continue by exploring some current applications and techniques
for measuring static surface tension.
Static surface tension applications
When a new surface forms, any surface active chemicals diffuse to that
surface and align. During this process, the surface tension is changing
rapidly and
continuously. Dynamic surface tension techniques are often appropriate to
track these changes. When the process reaches equilibrium, the surface
tension is
static and can be monitored by the techniques described below. However, there
are many applications where static techniques can be used to track surface
tension changes over time. One example is the tracking of time-release chemicals,
commonly employed in pharmaceuticals. Another application is to pre-determine
the location-specific, temporal-specific, and dose-specific release of surface
active chemicals to prepare a biological surface for the implant of devices
such as stents. Static surface tension monitors can be used to study the
release rates of these chemicals so that when these chemicals are
employed on the implant,
they are released effectively. Discrete static surface tension measurements
can be taken periodically over time, thus tracking relatively slow changes
in the properties of the liquid. A good analogy can be made to a time-lapse
movie of a flower opening, comprised of discrete frames capturing an image
at a specific time. A pure chemical or a solution in equilibrium is characterized
by a single static surface tension determination.
Static surface tension techniques
The Du Noüy Ring and Wilhelmy Plate techniques are gravimetric; frequently
both can be determined using a given commercial instrument. The classic measurement
technique, dating from the late 1800’s, is the Du Noüy Ring technique.
A precision-machined platinum/iridium ring, typically with a diameter about
2 cm, is suspended from a force measuring balance, then lowered into the liquid
and gradually withdrawn (in practice, the container of liquid is raised and
then lowered). As the ring is withdrawn, surface tension causes the liquid
to stick to the underside of the ring. The “weight” of the ring
increases due to the added weight of the adherent liquid. The maximum force
increase is a measure of the surface tension. Another related method is the
Wilhelmy Plate technique. A plate of metal is lowered until it just touches
the surface of the liquid. In this case the weight of the liquid that “crawls” up
the side (forming a meniscus) is the measure of surface tension. These
techniques are considered the most precise. However, care must be taken
with ring handling
and storage to avoid dimensional distortion. Further, contamination
of the plate can affect its wettability and therefore surface tension
measurement.
The pendant drop technique measures the shape of a liquid drop suspended
from a capillary needle. A liquid with high surface tension can become
quite elongated
before dropping off the needle. This is an optical measurement and
is best done using computer control and measurement. The technique
is not
as precise
as the force measurement methods because it depends on the eye of
the operator or the sophistication of costly detection hardware and
analysis
software.
However, several suppliers offer a system for both contact angle
measurement and surface
tension via the pendant drop method, potential spatial and cost advantages
to those who perform both determinations.
Another static technique, the spinning drop method, is particularly
suited to measuring low surface tension (down to micro-newtons/m)
that might
be below the limit of measurement for other techniques. In this
method a drop
of the
liquid to be analyzed is injected into a tube containing another
immiscible fluid of higher density. When the tube is spun about
its long axis, the
drop is forced to the center by centrifugal forces and its shape
elongates; the
degree of elongation is analyzed to give a measure of surface tension.
The authors acknowledge the helpful comments of Mark Coombs, Krüss
USA.
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical cleaning, precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
