## 5 Discussion  
Elution by magnetic agitation has been proved to be beneficial over the control case of no agitation, as shown in Figure 3. Two different mechanisms operate during the processes of elution by magnetic agitation. The magnetic impingement on the swab fibres causes the sample, absorbed in the swab, to be locally squeezed out at the point of impingement. This eluted sample is thoroughly mixed with the elution buffer due to the convective motion of the surrounding fluid, which is also caused by the motion of the magnets. Higher recovery efficiencies are obtained from physical impingement of particles on the swab tip, and hence is a better elution process than any technique without significant agitation. Unlike the no agitation case, where the time constant that is associated with the dissolution process is long and therefore only the sample at the surface layers of the swab get effectively eluted, under magnetic impingement, not only is the surface elution rapid, but the impingement also imparts forces to actively mix the sputum sample between the deeper layers of the swab and the surface layers that have absorbed the elution buffer from outside. This effectively helps in the dissolution of the sample that has been absorbed deep inside the swab matrix, which may not be eluted otherwise. The recovery efficiency of polystyrene beads from the swab fibres increases with an increase in rotational speed, as shown in Figure 5a. Multiple impingements on the swab fibres are responsible for these higher rates of elution. The magnetic particles rotate at \(2\times i\times N_{m}\), where \(N_{m}\) is the rotational speed of the motor and \(i\) is the gear ratio. In one complete rotation, the magnetic particles impart two impacts. Hence, the total number of impacts per minute becomes \(4\times i\times N_{m}\). As the rotational speed increases, the rate of impact on the swab fibres increases. The number of impacts at 500 RPM is 5 times higher than that offered at a low rotational speed of 100 RPM. Higher forces of impact between the particles and the swab provide better release of the samples due to efficient scrubbing action. Fresh elution fluid enters the fibre matrix after each subsequent impact and allows for the faster dissolution of artificial sputum. More particles can migrate into the elution volume, facilitating better recovery, and hence more beads are eluted from the swab tip. The acting forces on the swab were measured using a load cell, as shown in Figure 2b. The impact forces was found to increase with the increase in the number of impacts per minute at higher rotational speeds. The forces on the swab are mostly due to impact of the rotating magnetic particles, although the whole kinetic energy is not imparted to the swabs. A part of the kinetic energy is also dissipated in the fluid, which causes convective fluid motion around the swab. The duration of elution also determines the number of impingements that are caused. When the rotational speeds are kept constant, the total number of impacts per minute remains the same over the entire time period. At lower elution times, the total number of impacts (\(4\times i\times N_{m}\times t\)) has a significant effect on the number of eluted beads. For 30, 60 and 90 s, the increase in total number of impacts over time results in increased average impact force on the swab fibres, thereby increasing the number of recovered particles and the efficiency. Fresh fluid entering the fibre matrix helps in dissolving the artificial sputum solutions that pushes the beads into the elution fluid. But, the dissolution reaches a steady state after a very small amount of time, and after that, the impact is the only operating force that is acting on the swab. The impact force that is surrounding the swab removes the beads from the swab matrix, but only to a certain extent. After ~90 s, most of the beads that were free to move to the eluate had already migrated. The ones trapped deeper into the fibres do not get eluted and remains inside, even at higher elution times.