Introduction
Geometry can play a crucial role in the performance of fibres in different applications. Shape factors that influence performance include length (i.e. size of the longest dimension of the fibre), width (i.e. size of the shortest dimension), and curl. Despite the importance of fibre geometry, many conventional particle sizing measurements struggle to accurately capture the morphology of these particles. Volumetric-based particle sizing methods such as laser diffraction and Coulter Counters assume particles exhibit spherical geometry and only report equivalent spherical diameter (ESD) measurements. Manual microscopy, the primary method used for measuring fibre length and width, is low-throughput and labour-intensive to perform.
Flow imaging microscopy (FIM) instruments like FlowCam are an automated, high-throughput alternative to manual microscopy for fibre analysis. VisualSpreadsheet® software acquires and analyses images of fibrils, providing automated measurements of not only fibre length and width but also fibre straightness and curl from particle images similar to those obtained via manual microscopy (Figure 1). As FIM instruments capture fibre images in a flowing fluid, this technique offers much higher throughput than manual microscopy. These features make FlowCam an ideal instrument for rapid, automatic fibre analysis.
Fibre measurements in VisualSpreadsheet
Most particle imaging systems use Feret measurements to determine the length and width of particles. Feret measurements involve finding edges on opposite sides of a particle that are parallel to each other and measuring the distance between these edges. The shortest Feret measurement is reported as particle width, and the longest is reported as particle length (Figure 2). These Feret measurements are recorded as the “Length” and “Width” parameters reported by VisualSpreadsheet. While these measurements are accurate for symmetric and straight particles, Feret measurements dramatically undersize the length and oversize the width of curved particles.
VisualSpreadsheet also records Geodesic measurements of particle lengths and widths. Geodesic measurements account for the arcing of particles like fibres, thus providing a more accurate representation of fibre length and width (Figure 2). In VisualSpreadsheet, these fibre measurements are reported as geodesic length and geodesic thickness.
Figure 3 shows a comparison between these measurements for a straight fibre and for a curved fibre. Reported values for length (Feret) and geodesic length of the straight fibre are relatively similar, as are those for width (Feret) and geodesic thickness. When these values are compared for the curved fibre, the length (Feret) measurement is much lower than the geodesic length measurement, and width is a much larger value than the geodesic thickness measurement. While the length (Feret) measures the long-axis distance covered by the particle, the geodesic length factors the curvature of the particle into its reported length and is thus more accurate. Similarly, the geodesic thickness is more accurate as it primarily accounts for the width of the particle and not the short-axis distance covered by the particle.
Other fibre measurements available in VisualSpreadsheet include fiber straightness and fibre curl. Fiber straightness is the ratio of length (Feret) to geodesic length. Higher straightness values indicate better agreement between the Feret and geodesic length measurements, corresponding to straighter particle geometry. Fiber curl is calculated by dividing geodesic length by length (Feret) and subtracting one. A particle with a fibre curl of zero is perfectly straight and increasing curl values indicating higher degrees of curling. Figure 4 shows fibre measurement data for a curved wood fibre with a relatively high fibre curl value and relatively low fibre straightness value.
Using fibre measurements to evaluate samples
In applications where fibre morphology is important for quality control of fibrous materials, VisualSpreadsheet can be used to build and save custom filters that automatically report counts and concentrations of particles matching a particular specification. For example, if fibre straightness is of concern, pre-built filters can automatically report a percent of fibres that meet or exceed a user defined fibre straightness threshold.
Figure 5 shows data for custom value filters created for bleached softwood cellulose microfibrils at a specific stage of the refining process. For this example, at least 50% of the fibres must have fibre straightness ≥ 0.75 for a lot to pass quality control. After each lot of fibres is analysed, the filter bins instantly populate with a percentage of particles matching the passing criteria, allowing operators to quickly assess whether a particular lot has passed.
An added benefit of VisualSpreadsheet is the ability to directly interact with the filter grid and data plots. By selecting the “Pass – Fibre Straightness 0.75+” filter, particle images that match the filter will automatically display in the View Window (Figure 6, next page). These particle images can then be sorted, selected, and/or saved. Regions of histograms or scatterplots that contain particles matching the filter will also be highlighted. Data can be easily exported into Excel or as a PDF document for a seamless reporting and archiving process.
Conclusion
FlowCam is a powerful analytical tool that expedites and streamlines fibre analysis. Integrated fibre morphology parameters include geodesic length, geodesic thickness, fibre straightness, and fiber curl. Using these measurements, FlowCam provides more accurate and reliable data than volumetric-based methods and offers a significant time-savings over manual microscopy. The option of building custom filters in VisualSpreadsheet allows for instantaneous reporting of results at the conclusion of sample analysis, saving users time and effort in assessing fiber quality.
