Magnetic Propulsion of Self-Assembled Colloidal Carpets: Efficient Cargo Transport via a Conveyor-Belt Effect

We demonstrate a general method to assemble and propel highly maneuverable colloidal carpets which can be steered via remote control in any direction of the plane. These colloidal micropropellers are composed by an ensemble of spinning rotors and can be readily used to entrap, transport, and release biological cargos on command via a hydrodynamic conveyor-belt effect. An efficient control of the cargo transportation combined with remarkable “healing” ability to surpass obstacles demonstrate a great potential towards development of multifunctional smart devices at the microscale.

Figure 1

Schematics showing the procedure to assemble (left middle) and propel (middle right) a colloidal carpet. Assembly is induced by a magnetic field rotating in the particle plane $(x,y)$, whereas propulsion is induced by a rotating field in the perpendicular plane $(x,z)$ plus an oscillating component along the $y$ axis. (b) Snapshots of a colloidal carpet assembled and propelled by an applied field with amplitudes $H_0=1800\mathrm{A}{\mathrm{m}}^{-1}$, $H_z=1400\mathrm{A}{\mathrm{m}}^{-1}$, and frequencies $\omega =2{\omega }_y=942.5rad{s}^{-1}$, (Movie S1 in Ref. [32]). (c) Semilog plot of the average speed $⟨v_{\mathrm{cp}}⟩$ versus number of particles $N$ composing the carpet at different field values and frequencies (here, ${\omega }_y=\omega /2$).

Figure 2

(a) Snapshots in counterclockwise order, showing the guided motion of a colloidal carpet under dynamic magnetic fields with $H_0=2300\mathrm{A}{\mathrm{m}}^{-1}$, $H_z=1400\mathrm{A}{\mathrm{m}}^{-1}$ and $\omega =2{\omega }_y=942.5rad{s}^{-1}$ (Movie S2a). Center-of-mass trajectory is superimposed as a black line. (b),(c) Top, schematic; bottom, sequence of images of colloidal carpets moving against a $5\text{−}\mu \mathrm{m}$ immobile silica particle. In (b), the field parameters remain constant ($H_z=1400\mathrm{A}{\mathrm{m}}^{-1}$, $H_0=1800\mathrm{A}{\mathrm{m}}^{-1}$ and frequencies $\omega =2{\omega }_y=942.5rad{s}^{-1}$, Movie S2b), while in (c), the field $H_y$ is set to zero, and $H_z$ decreases to $H_z=560\mathrm{A}{\mathrm{m}}^{-1}$ after $t=26\mathrm{s}$ (Movie S2c). Later ($t=92.8\mathrm{s}$), the $H_y$ field is restored, and the carpet forms again. The scale bars in all images are $20\mu \mathrm{m}$.

Figure 3

(a) Sequence of images showing the transport of one yeast cell by combining translation and rotation of the carpet (Movie S3a). The position of the tracked cell is superimposed as a black line. The applied field parameters are the same as in Fig. 2. (b) Schematic of the hydrodynamic conveyor belt generated by the carpet. (c) Plots of the distance versus time for the carpet (filled squares) and cargo (empty circles) for Fig. 3 from $t=26.5\mathrm{s}$ to $t=41.0\mathrm{s}$. (d) Average speed of a colloidal cargo $⟨v_{\mathrm{cg}}⟩$ scaled with the carpet speed $⟨v_{\mathrm{cp}}⟩$ as a function of $\omega$ ($H_z=1400\mathrm{A}{\mathrm{m}}^{-1}$, $H_0=1800\mathrm{A}{\mathrm{m}}^{-1}$, and $\omega =2{\omega }_y$). Inset shows $⟨v_{\mathrm{cp}}⟩$ and $⟨v_{\mathrm{cg}}⟩$ separately.

Figure 4

Sequence of images showing the formation of two colloidal carpets from dispersed particles (top sequence) within a glass etched circular chamber ($115\mu \mathrm{m}$ diameter). The carpets are later transported across thin microchannels $20\mu \mathrm{m}$ width having (middle sequence), and finally the particles are redispersed by switching off the applied field 318.1 s later (Movie S4). The applied field parameters during transport are $H_0=1900\mathrm{A}{\mathrm{m}}^{-1}$, $H_z=980\mathrm{A}{\mathrm{m}}^{-1}$, ${\omega }_z=942.5\mathrm{Hz}$. The frequencies are ${\omega }_x={\omega }_z/2$ and ${\omega }_y={\omega }_z$ (${\omega }_x={\omega }_z$ and ${\omega }_y={\omega }_z/2$) when the carpets move upward (leftward).