Magneto-Responsive Shutter for On-Demand Droplet Manipulation

Abstract

Magnetically responsive structured surfaces enabling multifunctional droplet manipulation are of significant interest in both scientific and engineering research. To realize magnetic actuation, current strategies generally employ well-designed microarrays of high-aspect-ratio structure components (e.g., microcilia, micropillars, and microplates) with incorporated magnetism to allow reversible bending deformation driven by magnets. However, such magneto-responsive microarray surfaces suffer from highly restricted deformation range and poor control precision under magnetic field, restraining their droplet manipulation capability. Herein, a novel magneto-responsive shutter (MRS) design composed of arrayed microblades connected to a frame is developed for on-demand droplet manipulation. The microblades can perform two dynamical transformation operations, including reversible swing and rotation, and significantly, the transformation can be precisely controlled over a large rotation range with the highest rotation angle up to 3960°. Functionalized MRSs based on the above design, including Janus-MRS, superhydrophobic MRS (SHP-MRS) and lubricant infused slippery MRS (LIS-MRS), can realize a wide range of droplet manipulations, ranging from switchable wettability, directional droplet bounce, droplet distribution, and droplet merging, to continuous droplet transport along either straight or curved paths. MRS provides a new paradigm of using swing/rotation topographic transformation to replace conventional bending deformation for highly efficient and on-demand multimode droplet manipulation under magnetic actuation.

Keywords: droplet transport; intelligent surfaces; laser kirigami; magnetic actuation; wettability.

Figure 1

Fabrication and magnetic actuation behaviors of MRS. a) Illustration of the fabrication process of MRS. b) Images of the prestretched MRS. w and s are the width and the interval of microblades, respectively. d is the diameter of the cord. c) Schematic illustration of MRS rotation under magnetic actuation. α and β are the rotation angles of MRS and actuating magnetic field, respectively. θ is the tilt angle of MRS. d) Magnetic hysteresis loop of MRS. Insert is the magnetization profile of MRS. e) Images and FEA results of the swing and rotation process of MRS under magnetic actuation. f) The maximum rotation cycles of MRS with d of 0.07, 0.14, 0.20, 0.35, 0.47, and 0.68 mm under 60 mT magnetic actuation. g) The maximum rotation cycles of MRS with d of 0.14 mm under actuating field intensity of 25, 60, 105, and 130 mT.

Figure 2

Janus‐MRS with switchable wettability. a) Schematic of Janus‐MRS with switchable wettability under magnetic actuation. b) SEM images and corresponding water contact angles of the top surface and the bottom surface, respectively. c) Optical images demonstrate magneto‐responsive switchable wetting states between superhydrophobicity and hydrophilicity of Janus‐MRS (3 µL). d) The wetting stability of Janus‐MRS for 1000 cyclic switches under magnetic actuation. e) The droplet (9 µL) impact behaviors on Janus‐MRS with switchable wettability. The water droplet is bounced away from the superhydrophobic side while pinned on the hydrophilic side upon magnetic actuation.

Figure 3

Directional bounce and controllable transport of droplets on SHP‐MRS. a) Schematic of SHP‐MRS under magnetic actuation, whose surfaces are superhydrophobic. b) The bounce behaviors of droplets (6 µL) on SHP‐MRS at swing angles α of −50°, 0°, and 50°, respectively. c) The directional transport of water droplet (18 µL) on SHP‐MRS (w = 1.25 mm, and s = 0.30 mm) under a rotating magnetic field. d) The droplet (18 µL) moves back and forth on SHP‐MRS driven by the clockwise and anticlockwise rotating magnetic field, respectively. e) The effects of rotation cycles n on the transport distance, l, under w of 0.75, 1.0, 1.25, and 1.5 mm and s of 0.25, 0.30, 0.38, and 0.50 mm, respectively. f) The phase diagram revealing the transportation capability of SHP‐MRS under different widths and intervals of the microblades. The region (i) represents the failure of droplet transport in which droplets gets stuck between the intervals of the microblades, while the region (ii) represents the success of droplet transport. g) The fan‐shaped SHP‐MRS (w = 0.65–1.4 mm,and s = 0.30–0.50 mm) delivers the droplet (30 µL) in an arc pathway. h) Demonstration of SHP‐MRS (w = 1.25 mm, and s = 0.30 mm)‐ based microreactor. The CuSO₄ droplet (30 µL) is delivered to react with NaOH droplet, and blue Cu(OH)₂ precipitates are synthesized.

Figure 4

Multifunctional droplet manipulation using a slant LIS‐MRS. a) Schematic of LIS‐MRS under magnetic actuation, whose surfaces are slippery. b) Schematic illustration of droplet transport on the top surface, penetration through interval of microblades, and transport under the bottom surface of the slant LIS‐MRS. c) The continuous droplet (7 µL) delivery process. It is composed of the droplet transport on the top surface, the droplet penetration, and the droplet transport under the bottom surface of a slant LIS‐MRS (w = 1.25 mm and s = 0.30 mm). d,e) Five typical stages of droplet transport on d) the top surface and e) the bottom surface of slant LIS‐MRS, respectively. f) The relationship between droplet transport distance, l, and swing cycles, n, of actuating field. The widths of microblades, w, are 1.25 and 1.5 mm, respectively. g) The phase diagram revealing droplet transport behaviors of slant LIS‐MRS under different swing angles α. The regions (i) and (iv) represent that small droplets get pinned on the surfaces. The regions (ii) and (v) represent that the droplet can be successfully transported along LIS‐MRS. The region (iii) represent that the large droplet cannot be transported along LIS‐MRS. The region (vi) represents the droplet may fall off from LIS‐MRS. h) Droplet (12 µL) distribution using LIS‐MRS (w = 1.50 mm and s = 0.30 mm) for programmable chemical reaction. Droplets were one by one transported to the targeted positions on LIS‐MRS and distributed to react with the underlying droplets.

Figure 5

Droplets merging and droplet transport in the intervals of LIS‐MRS (w = 1.5 mm, s = 0.30 mm) using rotation actuation. a) Schematic diagram of droplets merging and droplet transport in the intervals of LIS‐MRS driven by a rotating magnetic field B. b) The detailed droplets (3 µL) merging process of LIS‐MRS. c) The detailed droplet transport process of LIS‐MRS. d) The effects of rotation cycles n on the droplet transport distance, l, under w of 1.25 and 1.5 mm and droplet volume of 3 and 6 µL, respectively. e) The phase diagram revealing droplets merging and droplet transport behaviors of LIS‐MRS under different microblade widths, w. The region (i) represents that the droplet is adhered to one microblade and cannot contact the adjacent one, the region (iii) represents that the droplet is too large to be delivered by the microblade. The region (ii) represents that the droplet can be successfully merged and delivered. The droplets merging and droplet transport back and forth along f) a horizontal LIS‐MRS, g) a slant LIS‐MRS, and h) a curved LIS‐MRS, respectively. i) The transport of droplet (4 µL) array along a LIS‐MRS.