Self-propelling colloids with finite state dynamics

Abstract

Endowing materials with the ability to sense, adapt, and respond to stimuli holds the key to a progress leap in autonomous systems. In spite of the growing success of macroscopic soft robotic devices, transferring these concepts to the microscale presents several challenges connected to the lack of suitable fabrication and design techniques and of internal response schemes that connect the materials' properties to the function of the active units. Here, we realize self-propelling colloidal clusters which possess a finite number of internal states, which define their motility and which are connected by reversible transitions. We produce these units via capillary assembly combining hard polystyrene colloids with two different types of thermoresponsive microgels. The clusters, actuated by spatially uniform AC electric fields, adapt their shape and dielectric properties, and consequently their propulsion, via reversible temperature-induced transitions controlled by light. The different transition temperatures for the two microgels enable three distinct dynamical states corresponding to three illumination intensity levels. The sequential reconfiguration of the microgels affects the velocity and shape of the active trajectories according to a pathway defined by tailoring the clusters' geometry during assembly. The demonstration of these simple systems indicates an exciting route toward building more complex units with broader reconfiguration schemes and multiple responses as a step forward in the pursuit of adaptive autonomous systems at the colloidal scale.

Keywords: active matter; microrobotics; microswimmers.

Fig. 1.

Schematic representation of the multistate dynamics of reconfigurable colloidal clusters. (A) Representation of the reconfiguration of G-PS and R-PS dumbbells upon crossing the respective transition temperatures T_{G ⇌ G′} and T_{R ⇌ R′} and schematic of the corresponding transition between the different states as a function of temperature T. (B) Extension of the previous concept to a three-state particle (R-G-PS trimer).

Fig. 2.

Two-state microswimmers. (A and B) Combined bright-field+fluorescence micrographs of G-PS (A) and R-PS (B) dumbbells in the sCAPA traps. The scale bars are 5 µm. (C and D) Schematic of the experiment for the two-state temperature-induced switching in the self-propulsion of G-PS (C) and R-PS (D) dumbbells. The curved black arrows schematically represent the EHDFs generated by the PS and microgel lobes, respectively, and the straight blue arrows indicate the net propulsion due to these EHDFs. Plasmonic heating of the substrate illuminated with a light of intensity I_{G ⇌ G′} (I_{R ⇌ R′}) raises the local temperature above T_{G ⇌ G′} (T_{R ⇌ R′}) inducing a phase transition in the microgels. Note that because T_{R ⇌ R′} >  T_{G ⇌ G′}, the corresponding illumination intensity is higher. (E) Mean self-propulsion velocity < v> as a function of temperature T for the R-PS (triangles) and G-PS (squares) dumbbells. The shaded bands are the 99% confidence intervals calculated over 350 and 70 particles for the R-PS and G-PS, respectively. The vertical dashed lines mark the transition temperatures for the green and red core microgels, respectively. The inset schematically shows the switching of the self-propulsion state upon crossing the critical temperature, taking an R-PS dumbbell as an example. The black curved arrows represent the EHDFs generated by the microgel and the PS particle, with characteristic velocities $U_{\mu gel}$ and U_PS, respectively, and the straight blue arrows indicate the net propulsion with velocity V.

Fig. 3.

Assembly of reconfigurable trimers. (A) Schematic showing the filling of the 3D-traps with sCAPA for a single colloidal 120-cluster. The numbers indicate the order of the deposition steps and the arrows respective deposition directions. (B–D) Combined bright-field+fluorescence micrographs of R-G-PS trimers in the sCAPA traps with 60 (B), 120 (C), and 180 (D) degrees between the microgels. Scale bars are 5 µm.

Fig. 4.

Three-state dynamics for G-R-PS 120-clusters. (A) Schematic illustrating the 120-cluster in state GR, G’R, orG’R’ depending on the illumination conditions. The black arrows represent the EHDFs and the blue ones indicate net propulsion. (B) Light exposure sequence (dashed line) over time and resulting temperatures (solid line) as predicted by finite element simulations. The color coding of the temperature corresponds to the color scale bar on the right of the graph. (C) Example trajectory of a G-R-PS 120-cluster color-coded with time and corresponding temperature as represented by the color scale bars on the left of the graph. This trajectory displays the closest resemblance to the average behavior of the whole data set. The scale bar is 20 µm. (D) Mean self-propulsion velocity (solid line) and rotational diffusion (box plots) of the microswimmers over time color-coded with temperature. The shaded bands are the 99% confidence intervals for the velocity calculated over between 100 to 124 trajectories. In the box plot, the colored line indicates the median, the size of the box indicates the interquartile range (IQR), and the whiskers delimit the range (Q1-1.5*IQR)–(Q3+1.5*IQR), where Q1 and Q3 delimit the first and third quartile of the data, and the dots mark the outliers.

Fig. 5.

Multistate motility of different active clusters. Each column corresponds to the 60-, 120- or 180-clusters, respectively. (A) Schematic of the three clusters in state (GR),(G’R), and (G’R’). (B) Fraction of trajectories that display clockwise chiral motion (CW, right-leaning dashes), counter-clockwise chiral motion (CCW, left-leaning dashes), or nonchiral active Brownian particle motion (ABP, no dashes) in (GR), (G’R), and (G’R’). The criterion for chiral motion is $⟨|\omega |⟩t>2\ast \sqrt{D_{r}t}$ for t = 30 s. (C) Box plot of the mean self-propulsion velocity in (GR), (G’R), or (G’R’). The box plots are calculated from between 52 to 79, 106 to 104, and 78 to 144 trajectories for the 60-, 120- or 180-clusters, respectively. In the box plot, the colored line indicates the median, the size of the box indicates the interquartile range (IQR), and the whiskers delimit the range (Q1-1.5*IQR)–(Q3+1.5*IQR), where Q1 and Q3 delimit the first and third quartile of the data, and the dots mark the outliers. (D) Ensemble-averaged MSD in (GR), (G’R), or (G’R’). Symbols: experimental data. Lines: computed MSD of for a chiral active Brownian particle with the ⟨|ω|⟩, ⟨v⟩, and ⟨D_r⟩ of each state (43).