Focusing particles (both biological and synthetic) into a tight stream is usually a necessary step prior to counting, detecting, and sorting them. The various particle focusing approaches in microfluidic devices may be conveniently classified as sheath flow focusing and sheathless focusing. Sheath flow focusers use one or more sheath fluids to pinch the particle suspension and thus focus the suspended particles. Sheathless focusers typically rely on a force to manipulate particles laterally to their equilibrium positions. This force can be either externally applied or internally induced by channel topology. Therefore, the sheathless particle focusing methods may be further classified as active or passive by the nature of the forces involved. The aim of this article is to introduce and discuss the recent developments in both sheath flow and sheathless particle focusing approaches in microfluidic devices.
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Acknowledgements This work was supported by NSF under grant CBET-0853873 with Marc S. Ingber as the grant monitor. The support from Clemson University through a startup package to Xuan, the Creative Inquiry Program, and the Research Investment Initiative Fund Program is also gratefully acknowledged.
- Particle focusing
- Sheath flow