Active Brownian motion tunable by light

Abstract

Active Brownian particles are capable of taking up energy from their environment and converting it into directed motion; examples range from chemotactic cells and bacteria to artificial micro-swimmers. We have recently demonstrated that Janus particles, i.e. gold-capped colloidal spheres, suspended in a critical binary liquid mixture perform active Brownian motion when illuminated by light. In this paper, we investigate in more detail their swimming mechanism, leading to active Brownian motion. We show that the illumination-borne heating induces a local asymmetric demixing of the binary mixture, generating a spatial chemical concentration gradient which is responsible for the particle's self-diffusiophoretic motion. We study this effect as a function of the functionalization of the gold cap, the particle size and the illumination intensity: the functionalization determines what component of the binary mixture is preferentially adsorbed at the cap and the swimming direction (towards or away from the cap); the particle size determines the rotational diffusion and, therefore, the random reorientation of the particle; and the intensity tunes the strength of the heating and, therefore, of the motion. Finally, we harness this dependence of the swimming strength on the illumination intensity to investigate the behavior of a micro-swimmer in a spatial light gradient, where its swimming properties are space-dependent.

Light-activated self-propelled colloids

“…The emergence of arrested phase owing to density-dependent mobility has been discussed theoretically in the context of bacteria by Tailleur & Cates [58]. The role of the self-trapping mechanism for the emergence of clustering was shown in the recent and remarkable experiments by Buttinoni et al [59], where they used 'large' 4 μm self-propelled carbon-coated Janus colloids, which self-propel under illumination in a near-critical water-lutidine mixture [60], and for which the caps can be optically resolved, indicating the direction of self-propulsion. They showed that the particles in a clusters are arrested, heads-on.…”

Mini and Micro Propulsion for Medical Swimmers

“…The authors studied the swimming behavior of the Janus particles in the patterned surroundings and demonstrated the steering capability with the arranged obstacles. Buttinoni et al [35] continued this study and showed a good direction control by changing the intensity of the green laser beam, which was used to generate the temperature difference in the Janus particle. The particle moved into the high light intensity area with the speed up to 0.7 μm/s.…”

“…Among the chemical micro swimmers, even though there are still debates on the mechanism [27], some devices utilize the bubble recoiling method to make momentum transfer by inertia propulsion [28][29][30][31]. The electric and thermophoresis methods may be categorized as viscous propulsion, as the active Brownian motion method may [32][33][34][35].…”

External control strategies for self-propelled particles: Optimizing navigational efficiency in the presence of limited resources

“…Illumination of the sample with visible laser light leads to absorption and subsequent heating of the carbon coating. When the temperature of the coating exceeds T C , the binary liquid becomes locally demixed, which leads to propulsion of the particle by self-diffusiophoresis [18,19]. For the illumination intensities I used in this work, the propulsion velocity v linearly increases with I [ Fig.…”

“…1(b)] above a threshold value I = 0.28 W/mm 2 . Below this value, the light intensity is not sufficient to induce demixing and no active motion is observed [18]. As the illumination source, we used a diode-pumped laser with wavelength λ = 532 nm.…”