Pdc 001 hp

Each substrate was cleaned in a plasma cleaner (Harrick Plasma, PDC-001-HP) for 1 min before its use in the experiments. The neodymium magnet was placed underneath the substrate in order to secure the ferrofluid that was to be added on top of the substrate. The ferrofluid was added using a syringe. Adding a specific volume to a known surface area allowed us to control the thickness of the ferrofluid layer. The thickness of the ferrofluid in our experiments was 300 μm. The surface area of the ferrofluid on the substrate is the same as the surface area of the top surface of the magnet.

The mixed base and curing agent of Sylgard 184 with a weight ratio of 10:1 was spin coated onto the PMMA mold at a speed of 400 rpm to obtain a uniform layer with a thickness of ca. 250 μm. After degassing and curing in the oven at 90 °C for 1 h, slowly peeling the cured PDMS layer off from the PMMA mold was followed by air plasma (PDC-001-HP, Harrick Plasma) treatment at 30 W for 10 min. Next, the PDMS layer was dipped in the APTES solution (concentration of 15 mM with 35.1 μL in 10 mL DI water) for 12 h to achieve strong adhesion followed by sputtering of 50-nm-thick Au (Quorum Q150R, Sputter Coater) as the bottom electrode.

The devices were fabricated in two stages as shown in the supplementary document. In the first stage, the fabrication of the silicon mould for polydimethylsiloxane (PDMS) was carried out at The Hong Kong University of Science and Technology Nanosystem Fabrication Facility (HKUST NFF) using standard lithography and deep reactive ion etching (DRIE). 3  $\mathrm{\mu }$ m HPR 506 was spun, exposed in Karl Suss MA6 and developed by the FHD5 developer. It was used as a mask for etching in the DRIE Etcher. The etching depth across the silicon wafer was 90  $\mathrm{\mu }$ m ± 2 $\mathrm{\mu }$ m with multiple measurements by Tencor P-10 Surface Profiler, but the height of the single pattern was about the same. The second stage was done in a typical laboratory environment. 10:1 PDMS of 50 g was poured onto the mould and cured in the oven at 80  ${}^{\circ }\text{C}$ for 24 h. The glass slide (SLItech, microscope slides, MS-13) was bonded to the PDMS after 3 min of plasma treatment in the plasma cleaner (Harrick, PDC-001-HP) followed by baking at 80  ${}^{\circ }\text{C}$ . After inlets and outlets drilling and tubing (Tygon Microbore Autoanalysis Tubing, GM-06419-03) connection, the junction sealing was achieved by optical adhesion (Norland optical adhesive 61) with 2 h of UV light exposure in the light curing system (DYMAX, Model 2000 Flood) followed by 8 h of baking at 80  ${}^{\circ }\text{C}$ .

The microfluidic device contains an array of straight channels, which was fabricated based on soft photolithography. Briefly, a silicon wafer was firstly spin-coated with a negative photoresist (SU-8 2025, Microchem, MA, USA) to generate a patterned layer 30 µm thick as the mold master. After exposure and development, the mold master was then silanized with vaporized (tridecafluoro-1,1,2,2-tetrahydrooctyl)-1-trichlorosilane (Sigma-Aldrich, St. Louis, MO, USA) in a vacuum desiccator > 12 h for more convenient release of PDMS from the mold. To fabricate the PDMS microchannel array, polydimethylsiloxane (PDMS, Sylgard-184, Dow Corning, Midland, MI, USA) prepolymer was prepared by mixing the monomer with the curing agent at a 10:1 volumetric ratio. After degassing, the prepared PDMS pre-polymer was then poured on the silicon mold and thermally cured in an oven at 85 °C for 4 h. Afterward, the fully cured PDMS was stripped off from the silicon mold, punched with holes for inlets and outlets, and bonded onto a glass slide (Citoglas, Nantong, China) by oxygen plasma (PDC-001 HP, Harrick Plasma, Ithaca, NY, USA).