The elasto-inertial focusing has been widely employed for various biomedical applications such as cell sorting, monitoring and stretching measurement . However, the widely-employed channel geometries have been limited to simple straight channels which commonly occupy a large footprint. The spiral channel, which can roll up a long channel (up to the order of 10 cm) in a small footprint (e.g., 1 cm2), has been regarded as a classical channel design in inertial microfluidics , but is rarely employed in elasto-inertial focusing due to the coexistence of inertial migration, Dean flow and viscoelastic effect. In spiral microchannels, the three above-mentioned effects may simultaneously affect the particle focusing at finite Reynolds numbers. As illustrated in figure 1(a), particles randomly-dispensed near the inlet would equilibrate at a specific lateral position under the competition of elastic force (FE), inertial lift force (FL) and Dean drag force (FD). Our previous work  explored the complex dependent of particle focusing patterns on flow rate and channel structure under the coupling of these three effects. However, to our best knowledge, the flexible control of particle focusing in spiral channels has not been reported.
In this work, we realized the control of particle focusing position in a compact spiral channel through adjusting the polymer concentration of viscoelastic fluids. It is found that the lateral focusing position away from the inner channel wall (Xeq) could be flexibly controlled via adjusting the concentrations of the selected Poly(vinyl pyrrolidone) (PVP) solutions (see figure 1(b, c)). At the high polymer concentration (8.0 wt%), the particles could be prefect single-line focused at the channel centerline at specific flow rates (see figure 2) due to the dominance of elastic force over inertial lift force and Dean drag force. The particles in previously-reported spiral inertial microfluidics equilibrate very close to the channel wall , which prevents the application of this technique for traditional optical interrogations due to the unavoidable scattering of optical signals at the wall interface. Therefore, this center-line focusing may serve as a potential pretreatment for microflow cytometry detection. At low PVP concentrations (i.e., 5.2 wt%, 3.6 wt% and 2.0 wt%), the particles were found to shift towards the outer channel wall (see figure 3), which enables the continuous particle concentration at a low energy consumption to be possible. The physics behind the controllable particle shifting via adjusting polymer concentration is the comparable competition between the involved three forces (elastic force, inertial lift force and Dean drag force). To better understand the effect of polymer concentration on particle focusing, we quantitatively measured the focusing widths (normalized with particle diameter) and the lateral focusing positions of particles. The measured results were plotted as a function of flow rate (see figure 4). It is obviously to found that the particle focusing would shift towards the channel centerline with increasing polymer concentration.