## 3 Results and Discussion
#### 3.1.1 Ablation Time
Some correlation exists between the nanoparticle mean diameter size and colloidal density (compare Figures 1 and 2). A higher colloidal density is generally observed at higher nanoparticle mean sizes. This can be attributed to the fact that larger nanoparticles are heavier and hence the colloidal density is higher. For instance, a colloid with a mean nanoparticle diameter of 50 nm will require twice the number of nanoparticles as a colloid with a mean diameter of 100 nm for it to have the same colloidal density. Comparing samples 26 and 27, the former has a larger mean nanoparticle diameter (132 nm) than the latter (95 nm). This led to a higher colloidal density for sample 26 (753 mg/mL) than for sample 27 (703 mg/mL). The same results are observed for other samples (e.g., samples 1-2-3-4-5-6, 8-9 and others). Thus, it can be generalised that a high colloidal density is caused by a large nanoparticle mean size. It is observed that some samples are not in agreement with this hypothesis (increase in nanoparticle mean diameter = increase in colloidal density)--for example, Sample 18. Sample 18 has a relatively low nanoparticle mean diameter (97 nm) yet with a high colloidal density. This can be explained by observing the UV-Vis data for this sample as shown in Figure 1; this sample has a high colloidal density due to the higher number of nanoparticles (high absorbance). The size distribution of this sample is positively skewed which means that is has a higher number of small particles than large ones. Other samples also exhibit low colloidal density but show a large nanoparticle mean size and low absorbance which is in agreement with the hypothesis (e.g., sample 13). The low absorbance signifies a low nanoparticle count and hence a low colloidal density. Additionally, the absence of colloidal filtration after PLAL experiments is a source of error during colloidal density analysis. Partially ablated powder particles (submicron particles) can be pipetted within the colloid for weigh measurements which raise the colloidal density. This is supported by observing the nanoparticle size distributions for sample 13 (see Supplementary Information Figure S1 whereby a small number of particles in the range of 300-600 nm are present (these particles are in the sub-micron range which is much smaller than the minimum size of the original powder (0.06 mm) which means that some powder particles were partially ablated/fragmented). SEM images of some samples also showed the presence of submicron particles which increase the colloidal density if unfiltered (see Figure 4f). It is worth noting that a significant number of nanoparticles is lost during filtration processes and the size of the filter may influence the result, which is why filtration was avoided. Figure 4: (**a**) Sample with bimodal nanoparticle size distribution after 2 min; (**b**) sample with normal nanoparticle size distribution after 5 min; (**c**) sample with positively skewed nanoparticle size distribution after 25 min; (**d**) effect of ablation time on nanoparticle size distribution. All samples are bimodal at 2 min, mostly skewed at 5 minutes and mostly normally distributed at 25 min; (**e**) typical UV–Vis curve for a magnesium colloid. A distinct peak is observed at a wavelength of about 204 nm. Effect of Ablation Time on Nanoparticle Mean Diameter  
The nanoparticle diameter tends to decrease with increasing ablation time from 5 to 25 min; seven of nine sample sets exhibited this trend as shown in Figure 5. This trend is observed for all samples at medium-high scan speeds (3250-3500 mm/s) and/or medium-high laser fluences (1.83-1.91 J/cm\({}^{2}\)). High scan speeds enable a fast nanoparticle fragmentation; thus, the fragmented particles increase with ablation time and scan speed. A high fluence encourages fragmentation even at low scan speeds due to the increased energy density. On the contrary, an increase in nanoparticle diameter (at low scan speeds) with increasing ablation time from 2 to 5 min is observed for some samples (e.g., samples 19-20, and samples 22-23).