The liquid droplet quartz crystal microbalance micro viscometer
Presented here is a novel use of the Quartz Crystal Microbalance QCM as a droplet
micro-viscometer. The droplet micro-viscometer is so named as it requires only a
micro litre of liquid in the form of a droplet in order to determine the liquid viscosity.
The crystal operates using the inverse piezoelectric effect and the droplet is applied to
one electrode of the freely oscillating crystal. Loading the crystal surface this way
produces a frequency change .11 in the oscillating frequency of the crystal, as a
function of the liquid viscosity. Monitoring of this frequency allows for determination
of the liquid viscosity.
Jointly sponsored by the EPSRC and British Nuclear Fuels plc, this research was
proposed by BNFL as a way of maintaining tighter control on an important parameter
of a solvent extraction nuclear fuel reprocessing process. Research in this area started
with the assessment of the QCM to give meaningful results for viscosity from a single
droplet of liquid, initially, alcohols whose physical parameters were well documented.
Encouraging results prompted further experimentation with tri-n-butyl phosphate or
TBP, diluted with odourless kerosene, OK.
Wider potential was realised and motor oils and fuel oils were targeted as a potential
use for this technology. Motor oil as viscosity is a first indicator of the oil
condition/quality and fuel oils specifically in the identification of red diesel when
diluted in white diesel.As this technology was intended for use as a sensor it was important, for financial
reasons, that the sensor element in contact with the liquid could be cleaned thoroughly
enough to allow re-use without degradation of the results produced. A cleaning
regime is covered in this work showing that cleaning is feasible but has limits in the
amount of times this can successfully be performed before a replacement QCM is
Some simple experiments on samples of whisky were performed in pursuit of a use in
the foodstuffs industry with the institute of food research. These show a possible
method of identification but no further work was done on this as other research
demanded the time.
Results show that the QCM results clearly demonstrate the use of a liquid droplet on
one face does produce meaningful results with relation to the viscosity of the liquid.
Various diJutions ofTBP and OK are distinguishable from each other as are various
dilutions of red diesel in white diesel and oils of varying viscosity.
The droplet QCM micro viscometer has the advantage over most other methods of
viscometry not in its accuracy of absolute viscosity measurement but in the very small
amount of sample liquid needed for test. This is a very desirable factor in situations
where repeat sampling of a fixed volume of liquid is required as the samples taken
need not significantly reduce the sample volume.