Magnetically Driven Manipulation of Nonmagnetic Liquid Marbles: Billiards with Liquid Marbles

Abstract

Liquid marbles are droplets encapsulated by a layer of hydrophobic nanoparticles and have been extensively employed in digital microfluidics and lab-on-a-chip systems in recent years. In this study, magnetic liquid marbles were used to manipulate nonmagnetic liquid marbles. To achieve this purpose, a ferrofluid liquid marble (FLM) was employed and attracted toward an electromagnet, resulting in an impulse to a water liquid marble (WLM) on its way to the electromagnet. It was observed that the manipulation of the WLM by the FLM was similar to the collision of billiard balls except that the liquid marbles exhibited an inelastic collision. Taking the FLM as the projectile ball and the WLM as the other target balls, one can adjust the displacement and direction of the WLM precisely, similar to an expert billiard player. Firstly, the WLM displacement can be adjusted by altering the liquid marble volumes, the initial distances from the electromagnet, and the coil current. Secondly, the WLM direction can be adjusted by changing the position of the WLM relative to the connecting line between the FLM center and the electromagnet. Results show that when the FLM or WLM volume increases by five times, the WLM shooting distance approximately increases by 200% and decreases by 75%, respectively.

Keywords: digital microfluidics; ferrofluid; liquid marble; liquid marble manipulation; magnetic manipulation.

Figure 1

Schematic of the apparatus.

Figure 2

(a) Sideview of the WLM and FLM with volumes of 150 μL and 250 μL, respectively. (b) The magnetic flux density versus the horizontal distance from the coil tip for coil currents of 1.5 A, 3.0 A, and 4.5 A. The vertical distance between the top of the coil and the plane on which FLMs were placed is fixed at 3 mm.

Figure 3

Schematic of the WLM shooting distance (${\mathrm{L}}_{\mathrm{WLM}})$ in addition to five parameters in this study, including the initial distances of WLM and FLM from the electromagnet (${\mathrm{D}}_{0,\mathrm{wlm} }$,${\mathrm{D}}_{0,\mathrm{FLM}})$, the WLM and FLM volumes (${\mathrm{V}}_{\mathrm{WLM}}$, ${\mathrm{V}}_{\mathrm{FLM}})$, and the coil current (I).

Figure 4

The process of launching a WLM with an FLM with volumes of 30 μL, initial distances from the electromagnet of 10 mm and 30 mm, respectively, and the coil current of 3 A. (i–iii) FLM moves toward the electromagnet while WLM is stationary. (iv, v) FLM impacts the WLM, and LMs stick and move together toward the electromagnet. (vi–vii) FLM stops at the top of the coil, while WLM continues its motion due to its high inertia.

Figure 5

(a) Position and (b) velocity of FLM and WLM versus time for liquid marble volumes of 30 μL, coil current of 3 A, and FLM and WLM initial distances of 30 mm and 10 mm from the electromagnet, respectively.

Figure 5

(a) Position and (b) velocity of FLM and WLM versus time for liquid marble volumes of 30 μL, coil current of 3 A, and FLM and WLM initial distances of 30 mm and 10 mm from the electromagnet, respectively.

Figure 6

WLM shooting distance versus WLM initial distance from the electromagnet for liquid marble volumes of 10, 30, and 50 μL, FLM initial distance of 30 mm from the electromagnet, and coil current of 3 A.

Figure 7

WLM shooting distance versus FLM initial distance from the electromagnet for liquid marble volumes of 10, 30, and 50 μL, WLM initial distance of 0 mm from the electromagnet, and coil current of 3 A.

Figure 8

WLM shooting distance versus WLM volume for WLM initial distances of 0, 10, and 20 mm from the electromagnet, FLM volume and initial distance of 30 μL and 30 mm from the electromagnet, respectively, and the coil current of 3 A.

Figure 9

WLM shooting distance versus FLM volume for FLM initial distances of 20, 30, and 40 mm from the electromagnet, WLM volume and initial distance of 30 μL and 0 mm from the electromagnet, respectively, and coil current of 3 A.

Figure 10

WLM shooting distance versus coil current for FLM initial distances of 20, 30, and 40 mm, WLM initial distance of 0 mm from the electromagnet, and liquid marble volumes of 30 μL.

Figure 11

(a) Schematic of a WLM placed with an off-center position from the connecting line between the FLM center and the electromagnet tip, leading to a nonstraight WLM motion path. (b) A sequence of pictures of the straight and nonstraight collisions between FLM and WLM with volumes of 20 μL, initial distances of 30 mm and 0 mm from the electromagnet, respectively, and coil current of 3 A. (c) WLM shooting distance versus WLM shooting path degree from the FLM path (the red line is fitted linearly to the experimental data.).

Figure 11

(a) Schematic of a WLM placed with an off-center position from the connecting line between the FLM center and the electromagnet tip, leading to a nonstraight WLM motion path. (b) A sequence of pictures of the straight and nonstraight collisions between FLM and WLM with volumes of 20 μL, initial distances of 30 mm and 0 mm from the electromagnet, respectively, and coil current of 3 A. (c) WLM shooting distance versus WLM shooting path degree from the FLM path (the red line is fitted linearly to the experimental data.).