An Open-Type Crossflow Microfluidic Chip for Deformable Droplet Separation Driven by a Centrifugal Field

Abstract

This study presents an innovative wedge-shaped inlet weir-type microfluidic chip designed to address common issues of clogging and inefficiency in microfiltration processes. Driven solely by centrifugal force, the chip integrates a crossflow separation mechanism and enables selective droplet sorting based on size, without the need for external pumps. Fabricated from PMMA, the device features a central elliptical chamber, a wedge-shaped inlet, and spiral microchannels. These structures leverage shear stress and Dean vortices under centrifugal fields to achieve high-throughput separation of droplets with different diameters. Using water-in-oil emulsions as a model system, we systematically investigated the effects of geometric parameters and rotational speed on separation performance. A theoretical model was developed to derive the critical droplet size based on force balance, accounting for centrifugal force, viscous drag, pressure differentials, and surface tension. Experimental results demonstrate that the chip can effectively separate droplets ranging from 0 to 400 μm in diameter at 200 rpm, achieving a sorting efficiency of up to 72% and a separation threshold (cutoff accuracy) of 98.2%. Fluorescence analysis confirmed the absence of cross-contamination during single-chip operation. This work offers a structure-guided, efficient, and contamination-free droplet sorting strategy with broad potential applications in biomedical diagnostics and drug screening.

Keywords: centrifugal microfluidics; droplet sorting; lab-on-a-chip; open crossflow filtration.

Figure 1

Schematic diagram of the experimental setup components.

Figure 2

Schematic illustrations of sample processing and sorting mechanism in the microfiltration chip: (a) Sample injection mechanism. (b) Cross-sectional view of the inlet region. (c) Diagram of droplet separation process. (d) Cross-sectional view of separation behavior.

Figure 3

Preparation of water-in-oil microdroplets via mechanical shaking.

Figure 4

Schematic of droplet dynamics and force interactions within the microfiltration chip under centrifugal field.

Figure 5

Effect of wedge angle on droplet separation performance at a slit depth of 100 μm: (a) Schematic of wedge angles. (b) Optical images of droplet separation outcomes. (c) Droplet size distribution and variation of separation threshold.

Figure 6

Effect of wedge angle on the separation efficiency of the microfiltration chip.

Figure 7

Effect of slit depth and slope angle on the droplet size sorting of oil-in-water emulsions: (a) slit depth of 200 μm; (b) slit depth of 150 μm; (c) slit depth of 100 μm; (d) slit depth of 50 μm.

Figure 8

Photographs showing the size distribution of oil-in-water emulsion droplets sorted by varying slit depths.

Figure 9

Influence of slit depth on the droplet size distribution of oil-in-water emulsions.

Figure 10

Comparison of theoretical and experimental critical droplet diameters under varying ramp angles and gap depths.

Figure 11

Synergistic regulation of sorting threshold in microfiltration chips by gap depth and ramp angle.

Figure 12

Comparison of droplet size distributions before and after sorting under different rotation speed conditions: (a) Size distribution of droplets under 0 rpm, dominated by capillary forces. (b) Size distribution at 100 rpm rotational speed. (c) Size distribution at 200 rpm rotational speed. (d) Size distribution at 300 rpm rotational speed.

Figure 13

Sorting efficiency and sorting limit of the chip at different rotation speeds: (a) Sorting efficiency of the chip at different rotational speeds. (b) Sorting threshold of the chip at different rotational speeds.

Figure 14

Performance of particle-encapsulated droplet sorting using oil-in-water emulsion structures.

Figure 15

(a) Azure 500 bioimaging system. (b) Fluorescence signal of cross-contamination detection without elution. (c) Fluorescence signal of cross-contamination detection with elution.

Figure 16

Verification of cross-contamination between dual fluorescently labeled samples: (a) fluorescence image under Rhodamine B excitation in the mixed sample; (b) fluorescence image under FITC excitation; (c) merged image of both fluorescent channels. The distinct separation of the two fluorescent signals indicates minimal cross-contamination within the chip.