Morphing of liquid crystal surfaces by emergent collectivity

Abstract

Liquid crystal surfaces can undergo topographical morphing in response to external cues. These shape-shifting coatings promise a revolution in various applications, from haptic feedback in soft robotics or displays to self-cleaning solar panels. The changes in surface topography can be controlled by tailoring the molecular architecture and mechanics of the liquid crystal network. However, the nanoscopic mechanisms that drive morphological transitions remain unclear. Here, we introduce a frequency-resolved nanostrain imaging method to elucidate the emergent dynamics underlying field-induced shape-shifting. We show how surface morphing occurs in three distinct stages: (i) the molecular dipoles oscillate with the alternating field (10-100 ms), (ii) this leads to collective plasticization of the glassy network (~1 s), (iii) culminating in actuation of the topography (10-100 s). The first stage appears universal and governed by dielectric coupling. By contrast, yielding and deformation rely on a delicate balance between liquid crystal order, field properties and network viscoelasticity.

Fig. 1

Nanoscale imaging of surface strains. a Schematic illustration of a laser speckle imaging (LSI) experiment. Photons from coherent plane-wave illumination, impinging on a LC device, are backscattered by TiO₂ pigments on the surface, and detected by a high-speed camera. Motion of the surface results in fluctuations in the detected speckle pattern, which we analyze to uncover the nanoscopic surface dynamics. b Schematic of the LC device prior to expansion (not to scale). Half of the mesogenic units (in dark grey) possess a permanent dipole moment which can be torqued by the field, whereas the other half (in light grey) serve as crosslinkers. c Surface profiles measured by digital holography microscopy (DHM): initial field-off state (black line), field-on steady state (red line), and relaxed field-off state (blue line). Switching turns the minima into maxima in a fully reversible way

Fig. 2

Spatial periodicity of surface motility. a Amplitude of the switching dynamics, probed with LSI, perpendicular (red line) and parallel (black line) to the electrodes. The motility is measured by 〈d₂(f = 1 Hz)〉, which is averaged over the orthogonal direction. The periodicity in d₂ matches the striated IDE pattern, as confirmed by corresponding power spectra (b): for the x-direction, a sharp peak is visible at 1/8.1 μm⁻¹, representing the distance between electrodes. By contrast, the motility along electrodes lacks clear periodicity

Fig. 3

Kinetics of surface deformations. Change in surface area upon switching the field on (a) and off (b), measured by the normalized change in scattering intensity, for field frequencies of 200, 300, 500 and 900 kHz. After an initial transient, the surface gradually expands (a) resp. contracts (b), exhibiting elastic ringing. Both the undulation frequency and final deformation amplitude increase with increasing field frequency. For clarity, the curves are offset vertically by multiples of 0.2. Inset: intensity structure function d₂ versus lag time τ. In line with elastic recoil, d₂ displays clear echoes (orange bullets), implying that the surface recurrently falls back to an earlier position. t − t_on = 100 s is chosen as reference point

Fig. 4

Frequency-resolved analysis of morphing kinetics. a Spectrogram of the dynamics after field switch-on at Δt = 0. Each horizontal strip represents one power spectrum of the temporal speckle intensity, averaged over 0.015 mm² surface. Three distinct dynamic stages can be identified. The colour scale ranges from −3 to −0.1. b Illustration of the field-off steady state (top) and morphing stages. Only the inter-electrode region is drawn. (I) Dielectric interactions drive the polar mesogens to oscillate along the AC field lines. (II) Under resonance conditions, the pendant and crosslink mesogens start moving cooperatively and plasticize the network. (III) A feedback loop of network weakening and increased oscillation amplitude causes amplification of the free volume, culminating in microscopic expansion. c Power spectra matching the indicated cross-sections in a. All stages exhibit high-frequency motions that are ballistic in nature i.e. characterized by power law −2. By contrast, a static reference sample (blue line) obeys a complex f-dependence dominated by low modes, due to external noise rather than intrinsic material properties

Fig. 5

Stage I: Dielectric response. Fast transient of nanoscale motility as the field drives the polar mesogens into oscillation, whose amplitude depends on the field frequency (a), voltage (b) and LC order (c). The error bars are defined as s.d. of the field-on/off average. Conditions: a homeotropic LCN at 70 V, b homeotropic LCN at 400 kHz and c 70 V and 900 kHz. d Superposition of all curves through normalization highlights the dynamic signature of this stage

Fig. 6

Stages II–III: Network plasticization and deformation. a In a homeotropic LCN (orange line), the emergence of concerted mesogen dynamics (stage II) leads to efficient surface motility (stage III), whereas an isotropic network (dark grey line) lacks the final deformation stage. b Peak motility in stage III (blue triangles, left ordinate) and % height change measured by DHM (magenta bullets, right ordinate) versus field frequency. Both techniques show an increase in surface expansion with increasing f_field, yet the relatively lower noise level of LSI compared to DHM (dashed lines) allows probing over an order-of-magnitude lower field frequencies (down to ~40 kHz versus ~800 kHz). c Field frequency sweep from 0 to 900 kHz at 0.9 kHz s⁻¹ for a homeotropic network (orange line) and isotropic network (dark grey line), confirming the extraordinarily high displacement sensitivity of LSI

Fig. 7

Chemical structures of the monomers used in the LCN syntheses