Results
Figure 3 shows the plots of the transmission and backscattering intensities against the position. The color-coding of the curves indicates the time at which they were recorded, from red (start of the experiment) to purple (end of experiment). Every curve represents one individual measurement.
The transmission diagram, shows a constant mean intensity value of ITr = 0 % with no changes throughout the whole experiment. This can be explained by the opaque appearance of the protein shake which prevents the transmission of any incident light.
The backscattering diagram, on the other hand, shows a clear mean signal between 2 mm and 55 mm of IBS = 22 % and a time dependent change of this signal. This becomes even more obvious once the change in intensity compared to the first measurement is being plotted (see Figure 4).
Fig. 3: Intensity vs. position diagrams of transmission (top) and backscattering (bottom)
The changes in backscattering intensity indicate that the protein shake dispersion destabilizes over the period of time it is measured. Thanks to the MSC software, it is possible to determine which mechanisms led to the destabilization during this experiment.
As shown in figure 4, the backscattering signal is being divided into 3 sections for analysis:
1st section 3 mm – 50 mm: Position independent increase of the backscattering intensity over time.
2nd section 1 mm – 4 mm: Time-dependent formation of a maximum in backscattering intensity.
3rd section 52 mm – 54mm: Decrease in intensity and signal shift towards lower positions.
Fig. 4: Backscattering intensity diagram relative to the intensity at t = 0 s
The 1st section is evaluated using the Value Analysis Method of the MSC software. The obtained diagram (see figure 5) plots the mean intensity of this section against time. A strong increase of the backscattering intensity can be observed during the first day.
From the second day onwards, a slight increase of the intensity can be detected, which is constant during the rest of the experiment with a rate of 0.04 % per day.
The strong increase observed during the first day of measurement is due to the decrease in air bubbles initially dispersed in the sample. These bubbles appeared during the shaking of the vial prior to the beginning of the measurement and then migrated towards the surface of the sample.
The slight but constant increase of the backscattering intensity observed in the 1st section affects almost the entire height of the sample. It can be explained by a change of particle size of one or more components, for instance through a process of agglomeration. The scattering capacity of particles is dependent, among other things, on their particle size. [2].
Fig. 5: Mean relative backscattering intensity of the 1st section vs. time (increase rate: 0.04 %/d)
In the 2nd section the Peak-Area Analysis method was used to evaluate the bottom part of the sample. The resulting diagram of peak area vs. time is given in figure 6 and shows similar trends to the ones observed in the 1st section (cf. figure 5).
Likewise one can observe a significant increase of the signal (area below the backscattering curves in the 2nd section) during the first day. The same assumption can be made in regards to the migration of air bubbles towards the top of the vial.
Throughout the rest of the experiment, the backscattering peak and its related area continue to increase. The same goes for the width of the peak, as seen in figure 4. This can be explained by the accumulation of sedimented particles at the bottom of the vial. This phenomenon increases the overall light scattering.
The analysis of the migration front obtained from the 3rd section the upper part of the sample confirms the assumption of a sedimentation process of single components (see figure 7).
Fig. 6: Peak area of backscattering intensity of the 2nd section vs. time (increase rate: 0.35 mm %/d)
According to figure 7, the migration front shifts only very slightly during the first three days. Between day 4 and day 7 it moves towards the bottom of the vial at a speed of 0.17 mm per day and across a distance of approximately 0.5 mm. This initial migration is explained by the separation of the components and the resulting sedimentation of particles.
A stable position of the migration front can be seen from day 7 to day 12 at a height of 53.0 mm. From day 12 onwards, the migration front moves once more towards the bottom of the vial at a speed of 0.12 mm per day.
From the previously described processes seen in figure 7, it is evident that there are no changes in intensity due to sedimentation during the first 3 days of measurement. The sedimentation process can be confirmed from day 4 onwards.
Along with the observations from the 1st section, it can be concluded that a sedimentation process is indeed occurring from day 4 onwards, once a critical particle size is reached, due to e.g. agglomeration, thus resulting in a shift of the migration front towards the bottom of the vial.
After day 7 the sedimenting particles have already sedimented below the 3rd section. Between day 7 and 12 only components of the mixture which are sedimentation resistant remain in the 3rd section.
But as seen in the analysis of the 1st section a global change in particle size still occurs. Hence, after day 12 another component has reached a critical particle size and another sedimentation process can be observed in the 3rd section.
Fig. 7: Changes in migration front position throughout the experiment