The analysis of fibrous particles has always presented a challenge. In systems that are volumetrically based like Coulter Counters, laser diffraction, and dynamic light scattering, Equivalent Spherical Diameter (ESD) is the only measurement provided, which is clearly not useful for fibers (example shown in Figure 1).
Due to this drawback, manual microscopy has been the main technique for the characterization of fibers where both width and length are analyzed. This process can be made dramatically quicker through the use of Flow Imaging Microscopy where computer algorithms are used instead of a human observer. Figure 2 presents the overview results of a FlowCam® characterization of industrial fibers.
Figure 1. How a volumetric-based system calculates ESD for a fiber: a 290 µm x 11 µm fiber is characterized as a sphere of diameter = 64 µm. Image Credit: Yokogawa Fluid Imaging Technologies, Inc.
Figure 2. Screenshot of FlowCam analysis of industrial fibers. Image Credit: Yokogawa Fluid Imaging Technologies, Inc.
This is comparatively simple to achieve in straight fibers, but becomes significantly more challenging in curled fibers. In a number of applications, measuring the extent of curl in the fibers can be essential to the performance of the end product. With its VisualSpreadsheet® software, the FlowCam delivers high-speed measurement of fiber width and length, along with characterizing fiber curl and fiber straightness.
Method
A sample of cellulose fibers suspended in acetone were analyzed using the FlowCam. The majority of imaging particle analysis systems utilize a measurement technique for width and length that is based on Feret’s diameter.
Feret’s diameter can also be called a caliper diameter as it is determined by the distance from two tangents created by the particle’s outline, similar to employing a caliper (seen in Figure 3).
Figure 3. Feret’s Diameter used to calculate length and width. Image Credit: Yokogawa Fluid Imaging Technologies, Inc.
VisualSpreadsheet and the FlowCam go beyond basic Feret measurements and also provide Geodesic Thickness and Geodesic Length as quantified measurements for every particle. Figure 4 displays the results for these two measurements for the same fiber particle image depicted in Figure 3 (particle #523 from the initial run as presented in Figure 2).
Figure 4. Geodesic Length and Thickness calculated by VisualSpreadsheet. Note the differences versus Figure 3. Image Credit: Yokogawa Fluid Imaging Technologies, Inc.
The image of fiber particle #523 from the initial run in Figure 2 is presented in Figure 5, along with the precise measurements for both the Geodesic-based calculations and Feret-based calculations as computed by VisualSpreadsheet.
As can be observed in Figure 5, VisualSpreadsheet includes two further calculations to describe the fibers: Fiber Straightness and Fiber Curl.
Fiber Straightness is determined by dividing the ratio of Length (Feret-based) by the Geodesic Length. A straightness value of one would describe a completely straight fiber, and as the fiber increases in complexity (non-straightness), this value is closer to zero.
Fiber Curl is determined by dividing the ratio of Geodesic Length by Length (Feret-based) minus one. A completely straight fiber would be defined by a value of zero, and increasingly larger values describe an increasing curl.
Contrast the measurements of the curled particle in Figure 5 with the straight fiber particle in Figure 6. As would be predicted, the curled particle has a lower ‘Fiber Straightness’ measurement and a higher “Fiber Curl” measurement when compared to the straight fiber. Also note that in the straight particle, the lengths and widths calculated by the two different techniques are much closer.
Figure 5. Curved fiber particle image with length and width measured using the Feret method and Geodesic method. Image Credit: Yokogawa Fluid Imaging Technologies, Inc.
Figure 6. Straight fiber particle image with length and width measured using the Feret method and Geodesic method. Image Credit: Yokogawa Fluid Imaging Technologies, Inc.
Results and Conclusions
In applications where fiber straightness is crucial in the final use of the fibrous materials, filters can be pre-built in VisualSpreadsheet to automatically report Fiber Straightness or Fiber Curl for the complete sample when the run has been completed.
For the specific material employed here, a sample was considered to pass if more than half of the fibers have a Fiber Straightness higher than 0.75. As can be observed in the summary statistics in Figure 7, this sample would (just) pass, containing fibers with a Straightness of >0.75 to a count percentage of 53.47.
Figure 7. Results of automated filters for fiber straightness on the overall fiber data from Figure 2. Image Credit: Yokogawa Fluid Imaging Technologies, Inc.
Lastly, in Figure 8, the images of those fibers that meet the specification can be presented for review just by double-clicking on the filter, which opens the View screen in VisualSpreadsheet.
Figure 8. Screenshot of FlowCam analysis of fibers, with images on right representing those with Fiber Straightness >0.75. Image Credit: Yokogawa Fluid Imaging Technologies, Inc.
FlowCam with VisualSpreadsheet is an innovative system for the efficient characterization of fibrous particles in relation to shape. In the application described here, more than 10,000 fibers were automatically characterized in a timeframe of just 23 seconds, generating data with significantly higher statistical confidence than what has been possible before.
This information has been sourced, reviewed and adapted from materials provided by Yokogawa Fluid Imaging Technologies, Inc.
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