## 3 Results  
### Morphological Characterization of CMFs  
Figure 2 shows the FESEM micrographs of the untreated ground RS, the lignocellulosic residue after the combined ultrasound-heating treatment, the bleached cellulose fibers, and the resultant CMFs obtained after the milling step. It can be observed that the untreated RS (Figure 2a) presented heterogeneous particle sizes, with a predominance of rod-like shape, as seen in the magnified FESEM image. The RS particles showed mean sizes of width and length in the 20-60 \(\upmu\)m and 16-270 \(\upmu\)m ranges, respectively. Prior to the RS fiber extraction, the grinding step was applied to improve the effectiveness of the combined ultrasound-heating method and the bleaching treatment. According to [PERSON] et al. (2016), RS has a strong crystalline structure, making the extraction and purification of cellulose fibers difficult. Likewise, RS presents a high proportion of non-fibrous cells, such as parenchyma, epidermis, and vessel cells (2016). The traditional alkaline pre-treatment, which aims to eliminate hemicellulose and lignin (2016), was replaced by a more environmentally friendly  
aqueous treatment based on the application of ultrasound, followed by reflux heating. Figure 2b reveals that the application of ultrasound and heating promoted a certain degree of fibrillar plane de-bonding and distorted the RS particles, as previously reported [8]. The acoustic cavitation produced by the ultrasound waves, combined with the thermal erosion generated by reflux heating, promoted the disruption of the primary structure of RS and improved the leaching out of non-cellulosic components from the cellulosic matrix [43; 44; 45]. As shown in Figure 2c, the bleached cellulose fibers exhibited a swollen, rough, and unregular appearance. The removal of non-fibrous components from the lignocellulosic matrix, such as hemicellulose, lignin, and waves, gave rise to an entangled material, as observed by other authors [46]. The lignocellulosic matrix comprises cellulose fibrils organized in fibers arranged in a longitudinal orientation and inserted in a cementing matrix consisting of hemicellulose and lignin [2]. Thus, after removing the amorphous components, the hydroxyl (-OH) groups of cellulose could establish extensive hydrogen (-H) bonds among their chains, leading to a more compact and disordered arrangement [47]. This phenomenon can be observed in Figure 2c in the FESEM image taken at higher magnification. Thereafter, the cellulose fibers were ground to break up the agglomerates and obtain thinner CMFs (Figure 2d). Image analysis using both optical microscopy and FESEM techniques served to determine the cumulative distributions of CMF lengths and widths, respectively shown in Figure 3. Therefore, the mechanical defibrillation of the cellulose fiber bundles was evident, which was considered to occur by detaching CMFs into long fibrils with a major cumulative frequency of lengths below 200 \(\upmu\)m (Figure 3a). Likewise, there was a marked reduction in the CMF thickness, showing predominant values between 5-15 \(\upmu\)m. The magnified image of the ground material, shown in Figure 2d, revealed that CMFs were isolated with a smooth surface pattern, confirming that the milling step was an efficient method to disrupt the tangled microfiber arrangement. In this regard, [PERSON] et al. [48] observed that mechanical defibrillation was effective at detaching cellulose fibrils from RS treated with the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)  
Figure 2: Field emission scanning electron microscope (FESEM) micrographs of untreated rice straw (RS) (**a**), RS treated with combined ultrasound-heating method (**b**), bleached cellulose microfibers (CMFs) before (**c**) and after (**d**) the milling step. Images were taken at 300\(\times\) with insets at 2000\(\times\). mediated oxidation method. The authors reported less uniform defibrillated particles, as also detected herein in Figure 2d.