Electrically activated ferroelectric nematic microrobots

Abstract

Ferroelectric nematic liquid crystals are fluids exhibiting spontaneous electric polarization, which is coupled to their long range orientational order. Due to their inherent property of making bound and surface charges, the free surface of ferroelectric nematics becomes unstable in electric fields. Here we show that ferroelectric liquid bridges between two electrode plates undergo distinct interfacial instabilities. In a specific range of frequency and voltage, the ferroelectric fluid bridges move as active interacting particles resembling living organisms like swarming insects, microbes or microrobots. The motion is accompanied by sound emission, as a consequence of piezoelectricity and electrostriction. Statistical analysis of the active particles reveals that the movement can be controlled by the applied voltage, which implies the possible application of the system in new types of microfluidic devices.

Fig. 1. Morphologies of liquid bridges under the effect of electric fields.

a Illustration of tunable morphologies of a ferroelectric nematic droplet. L and L_i represent cell gap and thickness of insulating layer, respectively. b Morphological phase diagram of ferroelectric liquid bridges as a function of voltage and frequency ($L$=12.4 µm and $L_i=750$ nm). The insets show the corresponding snapshots in distinct regimes. Circle symbols represent measurement points. c–j Images of active “febots” exhibiting translational motion with c–e horseshoe, f pear, g tuning-fork, and h trident shapes. The direction of motion is illustrated by red arrows. d Schematic illustration of the polarization structures along the branches, which cause repulsion between febots. e Time series snapshots of the motion and collision of two febots. Snapshots of rotating febots with i ring and j triskelion shapes. Yellow bars correspond to 100 µm length.

Fig. 2. Analysis of the motion of febots.

a Snapshot of the sample used for tracking febots. The cell was placed between two circular polarisers with opposite handedness. The inset shows the structure of the tracked horseshoe shaped febot. b The tracked trajectories after the starting points were shifted to (x, y) = (0, 0); each color corresponds to a different febot. c The time averaged velocity distribution at f = 6 kHz and U = 120 V. d The voltage dependence of the measured median velocity of horseshoe shaped febots at f = 6 kHz in a different cell. e Distribution of the mean orientation variations under 1 s.

Fig. 3. High speed imaging of ferroelectric nematic bridges excited by electric fields.

a Variation of $U/U_{\max }$ within one period with numbers (1–4) indicating the moments when the snapshots were taken. b, c: Images of the febot in one period of the applied AC voltage. b Inactive regime at 1 kHz and c active regime at 3 kHz. The waist of the meniscus is highlighted by green dashed line.

Fig. 4. Properties of sound emission from ferroelectric nematic bridges excited by electric fields.

a Spectra of the emitted sound for $f=1-10\mathrm{kHz}$ driving frequencies at U = 90 V. The vertical axis shows the Fourier amplitudes (A_FFT) in arbitrary units in linear scale. The spectrum measured at 6 kHz is magnified separately (with logarithmic scale). b Fourier amplitudes corresponding to the first and second harmonic signals as a function of frequency. c The voltage dependence of the first and second harmonic Fourier amplitudes at f = 6 kHz. Inset shows the magnified values corresponding to the first harmonic signal.