Microfluidic particle sorting has applications in diagnostics, chemical and biological analyses, food and chemical processing as well as environmental assessment. The main advantage of using microfluidic sorting platforms is the requirement of smaller sample volumes which, in turn, results in reduced costs and time. In diagnostics it reduces invasiveness for patients. Moreover, such miniature platforms can be massively parallelized, which makes higher number of particle sorting possible in less time.

Sorting can be done using a passive or an active technique. A passive technique does not require external fields and is solely based on the interaction between particles, flow field and the channel structure. An active technique, on the other hand, uses external fields such as magnetic or electrical. The animation below shows particle sorting of three particle species using a passive sorting technique based on the hydrodynamics of the microfluidic platform.

In this blog, I will discuss the physics behind the sorting technique shown above and the use of FLOW-3D in performing such simulations.

Physics Behind the Hydrodynamic Sorting Technique

This technique works on the principle that, in low Reynolds number regimes, particles will follow certain respective streamlines in the flow field based on their mass and diameter. Given that their masses are constant, particles with a smaller diameter experience less drag force, while the particles with a larger diameter, experience a larger drag force. This causes bigger particles to get easily carried away with the flow around them. Smaller particles and their trajectories are less affected by the hydrodynamic force.

When the diameter is the same for all the particle species, but their densities, and hence their masses, vary, we see a different behavior during particle sorting. Although the drag force is the same for the particles of the same diameter, the heavier particles are difficult to decelerate because they are under the influence of larger inertial force. On the contrary, lighter particles are easier to decelerate. Therefore, the outcome is that lighter particles get easily carried away with the flow around them and heavier particles stay their course.

FLOW-3D for Particle Sorting Simulations

FLOW-3D’s particle model makes it very easy to perform particle sorting simulations. The model has the option to set different particle classes such as marker, mass, fluid, gas or void particles. For this simulation, mass particles were used. A certain type of particle class can have different species based on their diameter and density. For example, in the animation above, there are three species for the mass particle class.

The dynamics of the mass particles can be controlled by properties like diffusion coefficient, drag coefficient, turbulent Schmidt number, and restitution coefficient. Mass particles can even be assigned thermal and electrical properties. These properties can be fully exploited if the user wants to study multiple forces acting simultaneously on the particles.


The image below shows results from two different simulation cases varying the mass and radius.

Microfluidics particle sorting hydrodynamics
Simulation results from two different variations in particle diameter and mass

The particles with a small diameter (blue in left pane) or less mass (green in right pane) get carried towards the top of the constriction and at the divergence follow the streamlines that diverge upwards. The particles with a larger diameter (green in left pane) or larger mass (blue in right pane) move towards the bottom of the constriction. Upon exiting the constriction, these particles follow the streamlines that diverge downwards.

Accurate numerical analyses of sorting micro-particles in microfluidics particle sorting devices based on channel geometry, particle parameters and flow properties can be used for better design of such micro-devices.

With FLOW-3D’s powerful particle model, it is easy to set up a microfluidics particle sorting simulation. The animation at the beginning of the blog shows clean sorting of different particle species and the collection of each species in different outlets. In the next blog on microfluidics particle sorting, I will talk about a sorting technique based on gravity separation.

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