## 4 Results
### Characterization of Magnetically Induced Mechanical Impingement  
#### 4.2.1 Effect of Rotational Speed of the Magnet and Elution Time  
Several experiments were performed, according to the procedure shown in Section 3.1, to examine the change in recovery efficiencies with varying rotational speed and elution time. Each dry swab weighing 0.41 \(\pm\) 0.07 g absorbs an average of 227 \(\pm\) 28 \(\upmu\)L of the sample when dipped into artificial sputum matrix loaded with particles (1 \(\times\) 10\({}^{9}\) particles/mL). Therefore, the number of particles that are absorbed in the swab was calculated to be (1.8 \(\pm\) 0.3) \(\times\) 10\({}^{8}\) particles/mL. Upon elution with magnetic agitation, the number of particles eluted increased with the rotational speed. At lower rotational speed of 100 RPM, the number of eluted particles were \(\sim\)4 \(\times\) 10\({}^{7}\) but it increased rapidly to about \(\sim\)7 \(\times\) 10\({}^{7}\) particles at 500 RPM, when the elution time was increased to 60 s, the number of eluted particles increased for each rotational speed. The total number of eluted particles increased to \(\sim\)6 \(\times\) 10\({}^{7}\) at 100 RPM and it gradually rose to \(\sim\)8 \(\times\) 10\({}^{7}\) at a higher speed (500 RPM). The corresponding change in recovery efficiency with rotational speed at same bead concentration (\(\sim\)10\({}^{9}\) particles/mL) is shown in Figure 5a. At lower elution time (30 s) and speed (100 RPM), the recovery efficiency (38.4%) was slightly better than the case with no-agitation (33.7%). But, with an increase in rotational speed, the recovery efficiency increased to 67.6% at 500 RPM, and then remained constant. With higher elution times, the number of eluted particles increase. At 90s elution time, the total number of eluted particles increased from \(\sim\)6.5-10 \(\times\) 10\({}^{7}\) within 100-500 RPM and then remained constant. Similarly, at 120 s elution time, the corresponding values are \(\sim\)8 \(\times\) 10\({}^{7}\)-10\({}^{8}\) within the same speed range. The corresponding recovery efficiency increased for every rotational speed when the elution times were increased. The highest recovery efficiencies were found at 120 s elution time, where the values increased from 44.6% at 100 RPM to 73.4% at 500 RPM and then remained constant. In this case, also, the average results between 100 RPM and the method of no agitation is not significant (\(p\)-value = 0.14), but becomes significant at 300 RPM (\(p\)-value = 0.0007). The changes in recovery efficiency with time for elution with a sample of 10% v/v (\(\sim\)10\({}^{9}\) particles/mL) concentration is shown in Figure 5b. The recovery efficiency increased with the elution time. In the control case, without magnetic actuation, there was a small increase in recovery with elution time (33.7-41% within 30 s-150 s). At low rotational speed (100 RPM), the recovery efficiency increased from 39.3% at 30 s to 48.6% at 120 s, and then remained steady. Higher elution time resulted in an increased efficiency, but with a less significant rate of increase than the effects that were seen with rotational speed. As shown previously, the recovery efficiency increases with the rotational speed. But at higher speeds, the recovery efficiencies also attain a steady value much faster. For example, at 400 RPM rotational speed, it remains horizontal at around 62%. At 500 RPM, the recovery efficiencies increase from 67.9% at 30 s to 71.2% at 90 s, and became constant afterwards. The effects of elution time on recovery efficiency is significant at lower speeds as compared to higher speeds. With lower rotational speeds (100 RPM), the recovery efficiency shows an increase of 9% (39% to 48%) over elution time (30 s-150 s). But, at higher rotational speeds, the slope of the curve decreases (only about 4% increase in recovery efficiency) and elution time does not have a pronounced effect on time. When considering both of these parameters, an operating condition of 500 RPM and 120 s are found to be suitable for maximum recovery.