Opto-thermoelectric microswimmers

Abstract

Inspired by the "run-and-tumble" behaviours of Escherichia coli (E. coli) cells, we develop opto-thermoelectric microswimmers. The microswimmers are based on dielectric-Au Janus particles driven by a self-sustained electrical field that arises from the asymmetric optothermal response of the particles. Upon illumination by a defocused laser beam, the Janus particles exhibit an optically generated temperature gradient along the particle surfaces, leading to an opto-thermoelectrical field that propels the particles. We further discover that the swimming direction is determined by the particle orientation. To enable navigation of the swimmers, we propose a new optomechanical approach to drive the in-plane rotation of Janus particles under a temperature-gradient-induced electrical field using a focused laser beam. Timing the rotation laser beam allows us to position the particles at any desired orientation and thus to actively control the swimming direction with high efficiency. By incorporating dark-field optical imaging and a feedback control algorithm, we achieve automated propelling and navigation of the microswimmers. Our opto-thermoelectric microswimmers could find applications in the study of opto-thermoelectrical coupling in dynamic colloidal systems, active matter, biomedical sensing, and targeted drug delivery.

Keywords: Applied optics; Optical manipulation and tweezers; Optical physics.

Fig. 1. Conceptual design for optical driving and steering of opto-thermoelectric microswimmers.

a Under light fields, PS/Au Janus particles are set to swim and rotate alternatively to follow a predefined path. b Upon light irradiation on a Janus particle, a temperature gradient ∇T pointing from the PS side to the Au side is generated on the particle surface due to the asymmetric absorption of PS and Au. c Once the Janus particle is dispersed in a 0.2 mM CTAC solution, a thermoelectric field is induced to drive the Janus particle along the temperature gradient. The white “+” symbols indicate the positively charged surface. In b, c, the asymmetric heating and thermoelectric field under a defocused laser beam are shown in the X–Z plane. d Schematic illustration and e asymmetric heating of the Janus particle when set to rotate (as shown by the maroon arrow) in the X–Y plane by another focused laser beam (indicated by the green region surrounded by a dashed circle). In d, e, the defocused laser beam is switched off

Fig. 2. Opto-thermoelectric swimming of PS/Au Janus particles under a defocused laser beam.

a Schematic illustration of the swimming mechanism. The velocity is directed from the PS hemisphere to the Au-coated hemisphere. b Swimming velocity as a function of the optical power for 5 µm PS/Au Janus particles. A 660 nm laser beam with a beam size of 31 µm was applied to drive the swimming. c Time-resolved images of a swimming 2.1 µm PS/Au particle. A 1064 nm laser beam with a beam size of 31 µm and a power of 32 mW was applied to drive the swimming. d Swimming velocity as a function of the optical power for 2.1 µm PS/Au Janus particles. Two different laser beams, i.e., a 1064 nm laser beam with a beam size of 45 µm and a 660 nm laser beam with a beam size of 45 µm, were applied to drive the swimming. The insets of b, d show a PS/Au Janus particle driven to swim under a defocused laser beam. All the aforementioned beam sizes were obtained by experimental measurement

Fig. 3. Orientation control of PS/Au Janus particles with a focused laser beam.

a Configuration and b corresponding dark-field image of a free 2.7 µm PS/Au Janus particle in the X–Z plane. c Configuration and d corresponding dark-field image of a rotating 2.7 µm PS/Au Janus particle in the X–Z plane. e Time-resolved dark-field images of the rotation of a 2.7 µm PS/Au Janus particle. The half-cyan, half-golden particles in the insets illustrate the corresponding configurations, while the maroon arrows in the insets illustrate the orientations. The green spot in the insets represents the laser beam (with a wavelength of 532 nm). f Displacement of the centre of the 2.7 µm Janus particle as a function of time. The centre of the beam is set as the origin of the coordinates. The fitting sinusoidal curves indicate a circular rotation. g Orientation evolution of the 2.7 µm Janus particle as a function of time. The fitting sawtooth wave indicates a consistent steering of the orientation. h Rotational rate as a function of the optical power for 2.7 µm PS/Au Janus particles. In a–d, for a free Janus particle, no boundary at the particle hemisphere was observed in the dark-field optical image because the Au-coated part tended to align with the direction of gravity. In contrast, when in-plane rotation of the Janus particle was initiated, the PS-Au interface became perpendicular to the substrate due to the coordinated effect of the thermoelectric force and the optical force. An asymmetric ring was observed in the dark-field optical image, with the brighter half-ring corresponding to the Au coating owing to its stronger optical scattering. The inset illustrates the rotation under a green laser beam (with a wavelength of 532 nm). The laser beam size on the sample plane is 2.65 µm for e, h. A power of 1.9 mW was applied for rotation in e

Fig. 4. Mechanism for orientation control of PS/Au Janus particles.

a Schematic illustration of a PS/Au Janus particle rotating under a focused green laser beam. b Simulated temperature profile and c force analysis of a 2.7 µm PS/Au rotating under a focused green laser beam. The orientation is parallel to the Y axis, and the offset between the centre of the particle and the centre of the beam is equal to 0.8 R. The laser beam propagates along the negative Z direction. The power of the green laser beam is set as 2 mW. d Calculated X and Y components of the thermoelectric force and optical force as a function of the offset (the origin is at the centre of the beam). The red dashed lines indicate the balance position X = 0.8 R. e Calculated thermoelectric torques in the balance position (X = 0.8 R) as a function of θ, which is the angle of the PS/Au interface relative to the substrate (X–Y plane). The insets of e depict the corresponding configurations and temperature profiles at certain θ (indicated by dashed lines or arrows)

Fig. 5. Directional swimming and targeted transportation of PS/Au Janus particles with a feedback control method.

a Schematic illustration of directional swimming with feedback control on the experimentally recorded images, where a focused green laser beam and a defocused red laser beam were employed for navigating and driving the swimming, respectively. b Flow chart of the feedback control method. c Optical setup and mechanical layout for the feedback control method. d Trajectories of 5 µm PS/Au Janus particles swimming in different directions. e Targeted delivery of a 5 µm PS/Au Janus particle to a 10 µm PS particle. A 5 µm 532 nm laser beam with a power of 2.6 mW was used to drive the rotation, while a 660 nm laser beam with a beam size of 31 µm and a power of 160–200 mW was applied to drive the swimming