Stability of solids dispersions

One  use of the ultrasonic SCA instrument is to measure the stability of different product formulations.  As an example of measurements on an unstable suspension, the figure below shows concentration readings of an ink suspension tested for a manufacturer of printing ink.  Repeated measurements were taken of the sample for the days indicated, up to 24 days.  For each measurement, the sample was placed in a water bath with the scanning mechanism and allowed to equilibrate to 35C.  After testing, the sample was returned to the shelf to settle further.

On the first day, the concentration was 16.5% all along the container.  The first sign of  settling was detected after just one day, with a 2.0 cm build-up of solids at the bottom of the sample.  After five days, a sharp interface in concentration began to appear about 1.3 cm from the bottom of the container.  Neither this interface nor the lower concentration at the top could be observed by the human eye.  The formation of the interface and its movement was governed by the particle size and viscosity of the slurry, with progressive movement of this interface toward the bottom of the container.  After 24 days, significant settling occurred, and the concentration at the bottom reached about 40%, with the top dropping to 11%.  By the fourth month, the settling was pronounced, with the bottom concentration at 50% and the top at 3%.  These ultrasonic measurements are helping an ink manufacturer ensure the shelf life of printing ink formulations.

Emulsion breaking and creaming measurement

The ultrasonic technique is also very useful for determining emulsion stability.   Early indications of phase separation or breakout can help dispersion manufacturers assure the shelf life of their emulsion products.  As an example of emulsion breakout, Figure 4 shows the ultrasonic time-of-flight (TOF) scans for a silicone oil-in-water emulsion over a period of 8 days.  Like the suspension measurements shown above, the sample was placed in the water bath, allowed to equilibrate to C, scanned, and then returned to the shelf to settle further.

The settling curves for the emulsion show a uniform oil concentration along the sample height on the first day.  However, after just one day the TOF at the bottom of the sample has decreased, indicating a high water concentration.   By the second day of settling, the emulsion has started to break out, and a higher TOF associated with the oil phase is clearly seen towards the top.  A layer is starting to form at 2.5 cm from the bottom of the container, but is not visible to the eye.  By the sixth day, the layer starts at 2 cm and is clearly visible.  The oil-water layer continues to move towards the bottom of the sample as the emulsion separates.  Another layer has started to form at 3.25 centimeters.  At the same time, the concentration of water at the bottom increases steadily even though a layer is never visible.