As a typical microfluidic cell sorting technique, the sizedependent cell sorting has attracted much interest in recent years. In this paper, a sizedependent cell sorting scheme is presented based on a controllable asymmetric pinched flow by employing an immersed boundarylattice Boltzmann method (IBLBM). The geometry of channels consists of 2 upstream branches, 1 transitional channel, and 4 downstream branches (Dbranches). Simulations are conducted by varying inlet flow ratio, the cell size, and the ratio of flux of outlet 4 to the total flux. It is found that, after being randomly released in one upstream branch, the cells are aligned in a line close to one sidewall of the transitional channel due to the hydrodynamic forces of the asymmetric pinched flow. Cells with different sizes can be fed into different downstream Dbranches just by regulating the flux of one Dbranch. A principle governing Dbranch choice of a cell is obtained, with which a series of numerical cases are performed to sort the cell mixture involving two, three, or four classes of diameters. Results show that, for each case, an adaptive regulating flux can be determined to sort the cell mixture effectively.
Sorting various categories of particles from the mixture to achieve pure sample is of great importance in biological and medical engineering. With the rapid development of micro total analysis systems, small sample volume, high throughput sample processing, high efficiency, and precise particle fractionation are several representative requirements to guide the design of sorting scheme [
In the present study, a numerical AsPFF cell sorter model is established with an immersed boundarylattice Boltzmann method (IBLBM), where the channel structure, the flow, the multiple sizes of cells, and their interactions are considered. Based on the model, cells with a prescribed size can be manipulated to enter a desired Dbranch simply by regulating the flux of one Dbranch (or the pressure of one outlet). The numerical results demonstrate that the numerical cell sorter is effective to perform an active and controllable cell sorting, which suggests an improved scheme of AsPFF and is valuable for guiding the experimental design of cell sorter on microfluidic chips.
In the numerical model, the fluid motion is solved by LBM with D2Q9 lattice model. The discrete lattice Boltzmann equation of a single relaxation time model is [
Once the particle density distribution is known, the macroscopical quantities, including the fluid density, velocity, and pressure, are then computed from
Although the lattice Boltzmann method is original from a microscopic description of the fluid behavior, the macroscopic continuity (
For the IBLBM frame, the fluid motion is first solved by LBM; then the position of immersed boundary can be updated within onetime step of
In (
The geometry model of for cell sorting is illustrated in Figure
The basic schematic structure of the simulated device.
The method and model are validated carefully here by performing a simulation of flow past a stationary circular cylinder. This simulation is carried out by employing IBLBM model. The computational domain is shown in Figure
The computational domain for flow past a stationary circular cylinder.
The results are shown in Table
Comparison of the flow characteristics.
Literatures 








 
Present  2.40  1.57  1.39 

0.160 
Reference [ 
2.59  1.58  1.39 

0.160 
Reference [ 
2.31  1.57  1.36 

0.163 
Reference [ 
2.35  1.66  1.38 

0.170 
Reference [ 
2.40  1.57  1.40 

0.162 
In order to actualize the pinched flow to sort cells, it is necessary to establish an appropriate pinched segment in the transitional channel, which is able to lead all cells to move along with the lower sidewall of the transitional channel. There are three aspects for establishing the pinched segment. First, the width
8
As shown in Figure
In our consideration, specific, multiple classes of cells with different sizes can be sorted if every class enters a Dbranch. In this section, the parameter
A Dbranch choice for a rigid circular particle can be predicted by the following experimental equations [
The predicted and numerical results of the choice of Dbranch which is related to the cell diameter and
Comparison of simulation and predicted outflow position.
According to the results, by regulating
The 16
The 20
As discussed in Section
Separation of differentsized cells: (a) separation of 8
A sizedependent cell sorting model with an asymmetric pinched flow is investigated numerically by immersed boundarylattice Boltzmann method. In the model, three aspects are summarized as the following. First, the geometry of the channels is designed specially according to the effective cell sorting, where the size of the transitional channel for controlling the pinched segment is discussed in detail. Second, the parameters
The authors declare that they have no competing interests.
This work is supported by the National Natural Science Foundation of China (no. 81301291) and the Beijing Higher Education Young Elite Teacher Project (no. YETP1208).