Abstract
The morphological properties of kaolin flocs were investigated in a Couette-flow experiment at the steady state under seven shear flow conditions (shear rates of 5.36, 9.17, 14, 24, 31, 41 and 53 s−1). These properties include a one-dimensional (1-D) fractal dimension (D1), a two-dimensional (2-D) fractal dimension (D2), a perimeter-based fractal dimension (Dpf) and an aspect ratio (AR). They were calculated based on the projected area (A), equivalent size, perimeter (P) and length (L) of the major axis of the floc determined through sample observation and an image analysis system. The parameter D2, which characterizes the relationship between the projected area and the length of the major axis using a power function, , increased from 1.73 ± 0.03, 1.72 ± 0.03, and 1.75 ± 0.04 in the low shear rate group (G = 5.36, 9.17, and 14 s−1) to 1.92 ± 0.03, 1.82 ± 0.02, 1.85 ± 0.02, and 1.81 ± 0.02 in the high shear rate group (24, 31, 41 and 53 s−1), respectively. The parameter D1 characterizes the relationship between the perimeter and length of the major axis by the function and decreased from 1.52 ± 0.02, 1.48 ± 0.02, 1.55 ± 0.02, and 1.63 ± 0.02 in the low shear group (5.36, 9.17, 14 and 24 s−1) to 1.45 ± 0.02, 1.39 ± 0.02, and 1.39 ± 0.02 in the high shear group (31, 41 and 53 s−1), respectively. The results indicate that with increasing shear rates, the flocs become less elongated and that their boundary lines become tighter and more regular, caused by more breakages and possible restructurings of the flocs. The parameter Dpf, which is related to the perimeter and the projected area through the function , decreased as the shear rate increased almost linearly. The parameter AR, which is the ratio of the length of the major axis and equivalent diameter, decreased from 1.56, 1.59, 1.53 and 1.51 in the low shear rate group to 1.43, 1.47 and 1.48 in the high shear rate group. These changes in Dpf and AR show that the flocs become less convoluted and more symmetrical and that their boundaries become smoother and more regular in the high shear rate group than in the low shear rate group due to breakage and possible restructuring processes. To assess the effects of electrolyte and sediment concentration, 0.1 mol/L calcium chloride (CaCl2) and initial sediment concentration from 7.87 × 10−5 to 1.57 × 10−5 were used in this preliminary study. The addition of electrolyte and increasing sediment concentration could produce more symmetrical flocs with less convoluted and simpler boundaries. In addition, some new information on the temporal variation of the median size of the flocs during the flocculation process is presented.
Highlights
When fine-grained sediment particles are transported in estuarine and coastal waters, they continually flocculate to form flocs of different sizes due to small-scale, particle−particle interactions
For the fractal flocs formed in a water-sediment mixture, properties such as volume, mass and density should be scaled as a power of their characteristic size (the exponent has been termed a three-dimensional (3-D) fractal dimension).this fractal dimension is a measure of how primary particles fill the entire space of the floc
In contrast to the floc size properties, we found that insufficient attention was paid to the morphological properties of the flocs at the steady state with respect to shear conditions
Summary
When fine-grained sediment particles are transported in estuarine and coastal waters, they continually flocculate to form flocs of different sizes due to small-scale, particle−particle interactions. Some flocs may break into smaller fragments/particles due to flow shear (breakup or disaggregation) [1,2,3,4,5], which results in flocs with varying sizes, excess densities and different setting velocities. For the fractal flocs formed in a water-sediment mixture, properties such as volume, mass and density should be scaled as a power of their characteristic size (the exponent has been termed a three-dimensional (3-D) fractal dimension).this fractal dimension is a measure of how primary particles fill the entire space of the floc. Similar to the 3-D fractal dimension, 2-D, 1-D and perimeter-based fractal dimensions have been defined and used to characterize the extent of the morphological irregularity of the floc boundary and/or its shape in the projected plane [7,8,9].
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