Frequency-Dependent Effects of Pulsatile Flow on Particle Inertial Focusing and Separation in Sinusoidal Microchannels

Inertial microfluidics leverages hydrodynamic forces to manipulate and separate particles at high flow speeds. Benefiting from its simplicity, high throughput, and accurate manipulation, inertial microfluidics has been extensively studied for processing particle and cellular samples. Most research in the field has focused on steady flows, where the flow speed and direction are constant. In real-world applications, pumping systems are never perfect and often experience flow variations due to device vibration, tubing compliance, or pressure fluctuations. Understanding the influence of flow pulsation on particle inertial focusing and separation is of great importance. Therefore, this work investigates the effects of flow pulsation on inertial focusing and separation in a sinusoidal microchannel. First, we analyzed the dynamic characteristics of the microfluidic system using a hydraulic-electric analogy and determined the critical frequency of the system (f_c) as ∼9 Hz. Next, we experimentally investigated the effects of pulsatile flows on the inertial focusing of microparticles and compared them with steady flows. Additionally, the effects of pulsation amplitude and frequency were studied, and the particle distribution in the time domain was also examined. Finally, we evaluated the influence of flow pulsation on the separation of a binary particle mixture. While particle focusing and separation remained stable at pulsation frequencies above the critical frequency of the system, they were negatively affected at lower frequencies. At 5 Hz, the purity decreased notably for 15 μm particles (from 94% to 71%) and slightly for 10 μm particles (from 99% to 94%), while at 10 Hz, purities remained close to the steady flow values. The results showed that the pulsation frequency was a more critical parameter than the pulsation amplitude for particle inertial focusing and separation performance.