MATERIALS AND METHODS
3. Introduction
3.2. Analyses
3.2.2. Particle Size Analysis
Several soil physical properties depend upon particle size distribution of soils. In fact, particle size distribution (PSD) is the basic determinant of almost all the physical phenomena. Efforts to employ this vital parameter to characterize a number of physical properties have gained momentum during the recent years.
3.2.2.1. Description of the instrument used and principle of analysis
Malvern Particle Analyser (Mastersizer 2000) (Figure 3.3) was used for the Particle Size Analysis (PSA). The standard result range for the Mastersizer 2000 is 0.02 to 2000 microns. It is capable of analyzing emulsions, suspensions or dry powders. The basic result produced by the analysis is a volume distribution which is the volume proportion within each size class of the total volume of the particles. The result can be edited at a later date by changing any of the parameters (e.g., refractive index) used during the measurement. The analysis is carried out with a combination of an accessory (e.g., Hydro 2000G), an optical unit and a stand-alone computer that runs the Malvern Software. The software controls the optical unit and the accessory and also analyses the raw data from the optical unit to give the size of the particles. Hydro 2000 MU (A), a sample dispersion accessory, is controlled from the key pad, not from the software.
The Malvern Particle Analyser works on the principles of Mie Theory. Mie theory
was developed to predict the way light is scattered by spherical particles and deals with the way light passes through, or is adsorbed by, the particle.
Knowledge of refractive index of the particle and its absorption is necessary for using this theory. When no published information is available, determining the most appropriate choice of refractive index is achieved by a process of looking at the fit and residual and selecting the refractive index that minimizes these values.
Figure 3.4: The Malvern Mastersizer 2000 used in Particle Size Analyses [Civil Engineering Department Laboratory, IIT Guwahati]
Size and other details about the structure of the particle dictate the way it will scatter light. Each size of particle will have its own characteristic scattering pattern, like a fingerprint, that is unlike any other size of particle. The Mastersizer uses the optical unit to capture the actual scattering pattern from a field of particles and calculate the size of the particles that created the pattern by using the Mie theory.
There are three distinct procedures involved in measuring a sample on the
z The sample is prepared and dispersed to the correct concentration and then delivered to the optical unit for measurement. This is done by the sample dispersion accessory. Values of pump speed and ultrasonic power is incorporated in the Standard Operating Procedure (SOP).
z The optical unit captures the scattering pattern from the prepared sample.
The detector array within the optical unit is made up of many individual detectors. Each detector will collect the light scattering from a particular range of angles. The detector array takes a “snap-shot” of the scattering pattern. For getting a representative reading of the scattering pattern, the Mastersizer makes over 2000 snaps for each measurement and averages the result. Each snap takes 1 millisecond. The detectors and windows should be kept clean at all times for correct results.
z When the measurement is complete, the raw data contained in the measurement are analyzed by the Malvern software.
Sample should be added one drop at a time. Care must be taken to ensure that the sample is thoroughly mixed. Bubbles interfere with the measurement. When the dispersant has been added to the accessory and circulated, it is advisable to switch off the unit for a brief period to allow trapped air to rise out of the system. There should be no twists or loops in the connected sample tubing. The pump and stirrer speed should not be so fast which can introduce air into the system.
Obscuration bar should be in the green region. The obscuration is simply the fraction of light “lost” from the analyzer beam when the sample is introduced. If the obscuration bar is in the green zone then the concentration is in the correct range. A mid
value in the green region is recommended. If it is in the red then the concentration is out of range. The instrument has wide range of concentrations that are acceptable and thus concentration do not have to be precise. A measurement should not be made until the obscuration has stabilized-indicating that the sample has properly dispersed. In general, enough sample should be added to give the following obscurations:
• 10% obscuration for very fine (up to 3-4 µm) materials
• 15% obscuration if the median size of the material is 50 µm or greater
• 25% obscuration if the material has a broad distribution or is coarse The measurement data from a particle field is contaminated by background electrical noise and also by scattering data from dust on the optics and contaminants floating in the “clean” dispersant. The “Measure background” facility makes a measurement of the system with only clean dispersant in as well as a measurement of the electrical background. This background information is subtracted from the sample measurement in order to “clean” the data.
3.2.2.2. Sample analysis
In the present study, Hydro 2000 MU (A) was the accessory used as dispersion unit for the particle size analysis. A beaker was used for holding the sample for dispersion and subsequent delivery to the optical unit. About 600 mL (for proper immersion of the stirrer) of deionised water (dispersant) was first placed in the beaker for measurement of background values. River water sample concentrated in suspended sediment was added to the beaker drop by drop after “Add sample” message appeared on the computer window. Sufficient amount of sample was added to bring the obscuration bar to the middle of the green region (about 15% obscuration). The pump speed used was
2500 rpm. No agglomeration of sediment was seen in the dispersed sample and so ultrasonic was not used in the analysis. The accessory and the sample holder (a beaker) were flushed between measurements (by drain button) to avoid cross-contamination of samples.
3.2.2.3. Report of the results
Particle size properties such as uniformity, specific surface area, surface weighted mean D[3,2], volume weighted mean D[4,3], d10, d50 and d90 are furnished in result analysis report in addition to the size distribution curve.