In and Out: Measuring Surface Tension: Part 3
Barbara Kanegsberg and Ed Kanegsberg, August 2003
In the previous column, we discussed some concepts and applications of static
surface tension. This month we consider dynamic surface tension. A static surface
tension determination provides the final score; dynamic profiling lets you
track the whole ball game. Put another way, static surface tension gives you
the final destination dynamic measurements tell you the course of the journey,
even some good stopping off points. Dynamic surface tension is important in
tracking changes during the fractions of seconds of surface formation, an important
consideration when surfactants are present. Dynamic measurements track from
time zero, the time of highest surface tension, when none of the surfactant
molecules have aligned at a surface, to equilibrium, the lowest surface tension,
when no additional surfactant molecules can be added to the surface. The shape
of the curves can provide insight to optimize wetting characteristics. Further,
by varying surfactant concentration, an additional variable is introduced,
enabling the more specific characterization afforded with three dimensional
analytical techniques.
Dynamic surface tension applications
People use dynamic surface tension characterization in diverse areas
One is formulation of aerosols optimize delivery of oral pharmaceuticals.
Designers
of aerosol dispensing systems may perform dynamic surface tension measurements
to optimize nozzle design. Dynamic measurements have even been demonstrated
as a tool for characterizing lung surfactants.
Dynamic measurements allow detection and tracking of multiple surfactants.
When two or more surfactants are present in a given solution, dynamic measurements
may fall and rise several times as heavier surfactants, that move to the
surface more slowly, dis- place lighter ones that get there first. One
equipment provider
detected atypical curves during a demonstration of what was stated to be
a blend containing a single surfactant. After ruling out equipment problems
and
replicating the results, it was discovered that one of the solvents in
the blend, presumed to be additive-free, actually contained surfactant.
Another application goes against the "mantra" that the lowest surface
tension is the best. An ink jet manufacturer had several ink formulations.
One of these exhibited a flashback into the node. After dynamic surface tension
measurements, it was concluded that the surface tension of this formulation
was too low; the droplet did not remain as a drop but retreated back onto the
node.
Dynamic surface tension technique
Dynamic surface tension is most commonly measured by the Maximum Bubble
Pressure method. If a probe consisting of a tube with an orifice is
lowered into the
test fluid and gas bubbles are forced through it into the liquid, the
maximum gas pressure, that corresponds to the moment that the bubble
breaks free,
is proportional to the surface tension. Because the pressure is also
affected by such parameters as the depth of immersion and viscosity
of the fluid
frequently two or more probes with differing orifice sizes are employed
in a differential
manner so that these competing effects can be cancelled in the measurement.
How does this technique work dynamically? By varying the bubble rate,
the time from surface generation (when the bubble first emerges from
the orifice)
to
bubble release (the time of the measurement) can be varied. Thus
one can trace the evolution of the surface tension from shortly after
surface
formation
(measured
with a rapid bubble rate), when there is minimal surface at the surface,
to a time long enough for surfactant molecules to diffuse to the
surface and align
themselves with hydrophobic ends pointing out of the liquid and lowering
the dace tension (using slow bubble rates). In other words, the surface
tension can be measured as a function of surface age, the time since
formation of
the
surface. These times can be as short as a few milliseconds or as
long as over 100 seconds (to measure equilibrium or static dm tension).
Competing effects
due to multiple surfactants can cause the surface tension to decrease
with
surface age, then increase, and then decrease again as one surfactant
reaches the surface only to be displaced and replaced by another.
The authors acknowledge the helpful comments of Victor Janule, Sensadyne
Instrument Division, Chem-Dyne Research Corp.
Victor P. Janule, Fingerprinting Surfachnts Using Dynamic Surface
Tension Measurement (Pharma-Chem; Scheduled for publication, June
2003)
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.
