## 3 Results and Discussion
#### 3.1.1 Ablation Time  
Three ablation times (2, 5, 25 min) were studied and their effects on UV-Vis absorbance (nanoparticle yield), colloidal density (mg/mL) and nanoparticle mean diameter are examined in the sections that follow. Samples are categorised in sets of 3 (samples 1,2,3 in one  
Figure 3: Mean colloidal density (mg/mL) per sample group (1–27) (95% CI). Figure 2: Mean nanoparticle diameter per sample group (1–27) (95% CI). set, 4,5,6 in another set, etc.) such that the only variable in each set is the ablation time. This enables us to individually study the ablation time while also observing the effects of other parameters (refer to Table 1 for the DOE). The ablation time influences the nanoparticle yield and diameter. The colour and colour-intensity of the resulting nanocolloid depends on both the nanoparticle mean size (due to plasmonic effects) and nanoparticle yield (population of nanoparticles). Effect of Ablation Time on UV-Vis Absorbance (Nanoparticle Yield)  
All UV-Vis curves for the 81 samples showed a similar distinct large peak 204 nm and differed in absorbance. The difference in absorbance at this peak was used to compare the samples. A typical UV-Vis curve of a magnesium colloid is shown in Figure 4e. The absorbance increases linearly with ablation time (Figure 1), regardless of the changes in other processing parameters (fluence and/or scan speed). This is an important property to discover, thus we can conclude that when a high nanoparticle yield is required without a major concern about nanoparticle size, a long ablation time is preferable, keeping in mind that a limit exists before shielding effects slow down ablation efficiency significantly. The average absorbance at 2, 5 and 25 min ablation time are 0.43, 0.70, 1.94, respectively. The average rate of change in absorbance between 2-5 min and 5-25 min ablation times are 0.089 and 0.062 a.u/min, respectively; thus, the ablation rate decreases with increased ablation time. This is attributed to shielding effects that cause primary/initial nanoparticles to absorb incoming laser photons, thereby shielding the laser from interacting with the powder to fabricate new (primary) nanoparticles. It is worth mentioning that the shielding effects reduced the ablation efficiency by about 30% when moving from 2-5 to 5-25 min, which is a promising result for batch mode PLA; the significance of this result is that we can still produce more nanoparticles at a decent rate even after 25 min of laser ablation without any flow of liquid required. This was supported by the high scan speeds (3000-3500 mm/s), large ablation paths supported by the spreading of the powder (55 mm outer diameter) and large hatch spacings (50 um). Additionally, powders reduce energy losses due to increased surface area, thereby increasing ablation efficiency. Effect of Ablation Time on Colloidal Density (mg/mL)  
Another indicator for nanoparticle yield is colloidal density (mg/mL). A volume of 1 mL of each sample was weighted and the density was calculated according to Equation (1). Colloidal density per sample is shown in Figure 2; a general increase in in colloidal density with time is observed from 2 to 5 min of ablation time (six of nine sample sets exhibited this trend, see samples 7-8, 10-11, 13-14, 16-17, 22-23, and 25-26) with some samples showing a decrease (three of nine sample sets, see samples 1-2, 4-5, and 19-20). Similarly, a general increase in colloidal density is observed with an increase in ablation time from 5 to 25 min (six of nine sample sets exhibited this trend, see samples 2-3, 11-12, 14-15, 17-18, 20-21, and 23-24). In all cases (nine of nine sample sets, thus samples 1-2-3, 4-5-6, etc. ), the rate of change of colloidal density with respect to time from 2 to 5 min is different to that from 5 to 25 min (magnitude and/or direction), signifying a shift in ablation efficiency. We can conclude that there is some correlation in colloidal density and ablation time; however, time is not the only factor affecting the density.