Several experiments were carried out using the MOFF device shown in figure 1A (with a height of 20 μm). Since the purpose of the experiments was to demonstrate the effects of particle concentration in focusing performance, the dimensions of the device were kept constant in all experiments. Density-matched aqueous suspensions of polystyrene particles (with diameters of, 5, 8 and 10 μm) were used in the experiments. The two inlets of the device were used to pump in two different solutions: the particle-laden suspension and buffer (used to dilute and fine tune the particle’s concentration on-chip). This allowed the rapid exploration of a diversity of concentrations and Re regimes, by simply changing the flow rate ratio between the two solutions while maintaining the total flow rate. The particles were imaged in a 100 μm-wide section immediately downstream from the last orifice.
The first relevant observation we present is in terms of the focusing mode observed in our experiments. At low and intermediate Re, the tendency of the particles is to focus at the center of the channel, while at larger Re the focus tends to happen laterally (at the two opposite extremes of the channel width). This is in direct contrast with previous reports of MOFF devices, where lateral focusing occurs at lower/medium Re and central focusing at higher Re. Although it is beyond the scope of this work to discuss it here, we presume that this discrepancy possibly arises from the ratio between the particle diameter and the orifice size, which is much higher in our device than in other MOFF devices characterized before.
We next show that particle concentration also has an effect on the focusing regime. This is illustrated in figures 1B, C and D, for particle diameters of 5, 8 and 10 μm, respectively, at different concentrations and flow rates. As it can be seen in all cases, the higher the concentration of particles is, the earlier (i.e. at lower Re) the transition toward lateral focusing begins to occur (with frequency peaks in the histograms appearing at the extremes of the chart). This effect of concentration on the transition from mid-channel to lateral focusing is best illustrated for 10 μm particles flowing at a total flow rate of 10 μL/min (Fig. 1D). At the lowest particle concentration, a very tall and narrow peak at the center of the device is obtained, while the highest concentration two small peaks begin to arise at the edges of the device.
A final relevant phenomenon that was observed is that the focusing performance of the MOFF device in the central focusing mode is very good (i.e. very narrow and tall frequency peaks at the middle of the channel) when the concentrations are lowest (ideally under 2 M/mL). This is apparent in all cases studied, but is more clearly seen, again, for the case of 10 μm particles at 10 μL/min. The higher the concentration is, the shorter and broader the frequency peaks at the mid-channel, until at the highest concentration that peak disappears and only two small peaks can be seen at the laterals of the channel. Particularly, when compared to the 8 or 5 μm particles cases, the mid-channel focusing performance with 10 μm particles at low concentrations is much better: a frequency peak over 0.7 is achieved with 10 μm particles, while the highest peaks observed for 5 and 8 μm particles are always under 0.5. This is likely because the lowest concentrations studied in 5 and 8 μm cases were substantially higher than the lowest concentrations of 10 μm particles (which were, indeed very low), and the optimum conditions for mid-channel focusing were never reached in these cases.
Figure 1E shows how at higher particle concentrations particles begin to crowd in the orifice sections. Together with the previous observations, this indicates that in conditions where particles cannot interact with each other (given the scarcity of neighbors) the focusing at the mid-channel is preferred at low and medium Re regimes. At the same regimes, though, when particle-particle interactions are common place, lateral focusing is induced. Therefore, a key mechanism that governs both the mode of focusing and its quality is likely to arise from particle-particle interactions, which seem to disrupt the performance of the mid-plane focusing mechanism while increasing the likelihood of particles to focus laterally instead.
