Session S59: Actuation in Soft Matter II

Liquid Crystal Elastomer Micron-Rings with Pre-Designed Molecular Orientations

Mechanical responses of liquid crystal elastomers (LCEs) to external stimuli are pre-programmable by predesigning molecular orientations, promising applications such as sensors and actuators and micro robotics. Recently we have developed capabilities to photopatterning arbitrary molecular orientations with high spatial resolutions and to fabricating LCEs with well-defined geometric shapes and predesigned 3D molecular orientations. In this talk, we will present studies on LCE micro-rings, especially on how their mechanical deformations and dynamics are affected by their molecular orientations and geometric shapes.   

Topography from topology in liquid crystal elastomer coatings

Liquid crystal elastomer (LCE) free-standing films containing topological defects deform on heating into three-dimensional shapes [1-3]. Attaching such LCE films to a rigid substrate as a coating provides the opportunity to create temperature-responsive surface topography, a phenomenon demonstrated by Babakhanova et al [4]. Using finite element method (FEM) simulation, we model surface deformations produced by LCE coatings with defects as a function of both topological charge and orientational phase angle. We find that, on heating, a radial +1 defect produces an inward depression while a circular +1 produces an outward elevation. The -1 defect and higher order +/- integer defects produce wrinkle patterns with out-of-plane surface deformations. By contrast, +/- half-integer defects give rise to both out-of-plane and in-plane displacements. Theoretical calculations in the limit of small strain elasticity explain these results and are compared with both FEM simulation and experimental data. [1] McConney et al, Adv Mater 25, 5880 (2013); [2] Konya et al, Front Mater 3, 24 (2016); [3] Modes et al Phys Rev E 81, 060701R (2010); [4] Babakhanova et al Nat Commun 9, 456 (2018).   

Multi-responsive polymeric microstructures with encoded pre-determined and self-regulated deformability

Dynamic functions of biological systems often rely on arrays of actively deformable microstructures undergoing a huge range of pre-determined and self-regulated reconfigurations and motions. Here, we introduce stimuli-responsive microstructures based on liquid crystalline elastomers (LCEs) that display a broad range of hierarchical, even mechanically-unfavored deformation behaviors. Using patterned magnetic fields during polymerization, we encode any desired uniform mesogen orientation into the resulting LCE microstructures, which is read-out upon heating above the nematic-isotropic transition as a prescribed deformation. By further introducing light-responsive moieties, we demonstrate multi-functionality of the LCEs with three actuation modes controlled by different external stimuli. We finally create patterned arrays of microstructures with encoded area-specific deformation modes and show their functions in responsive release of cargo, image concealment, and light-controlled reflectivity. We foresee that this platform can be widely applied in soft robotics and smart buildings.   

Electromechanical actuation of dielectric liquid crystal elastomers for soft robotics

Liquid crystal elastomers, networks of anisotropic molecules, are two-way reversible shape memory polymers. They have long been considered as intelligent materials reminiscent of biological muscles with orientational order. Despite significant developments in chemistry, processing, and handling methods of liquid crystal elastomers, most demonstrated actuation mechanisms still rely on thermal or optical stimulation, which often suffers low efficiency of energy conversion into useful work. Here, we report fast and efficient electrical stimulation of liquid crystal elastomer actuators with high output work density for potential soft robotics applications. Different from conventional dielectric elastomers, which often require prestrain, the intrinsic elastic anisotropy in liquid crystal elastomers allows us to design complex patterns of locally aligned liquid crystal molecules. In turn, we demonstrate pre-programmed twisting, bending, and other actuations.   

Programmed anisotropic transformations of cellular structures

The physical properties of metamaterials are controlled by the underlying material properties (molecular scale) as well as the design of the periodic structures, including shape, geometry, size, orientation, and arrangement (unit cell scale). Harnessing instabilities in soft materials based periodic structures offers an effective strategy to achieve multifunctionalities. Here, we report the programmable anisotropic transformations of cellular structures made of liquid crystalline elastomers (LCEs) on rigid substrates. By programming the mesogen alignment of liquid crystals (LC) using magnetic fields at the molecular scale, the desired anisotropies are introduced to periodic structures with different geometric designs at the unit cell scale. We demonstrate that the coupling of the two scales of controls leads to various novel deformation modes benefiting from both the mechanical guiding and the chemical guiding, which greatly broadens the design space of metamaterials. The design principle for programmed reconfigurations with prescribed anisotropies in periodic lattices is validated by both mechanics model and finite element simulation.   

Bioinspired design of vascular artificial muscle

Recently, liquid crystal elastomers (LCEs) have drawn much attention for its wide applications as artificial muscle in soft robotics, wearable devices and biomedical engineering. One commonly-adopted way to trigger deformation of LCEs is using embedded heating elements such as resistance heating wires and photo-thermal particles. To enable the material to recover to its unactuated state, passive and external cooling is often employed to lower down the temperature, which is typically slow and environmentally sensitive. Here, inspired by biology, we design and fabricate a vascular LCE-based artificial muscle (VLAM) with internal fluidic channel in which we inject hot or cool water to heat up or cool down the material to achieve fast actuation as well as recovery. We demonstrate that the actuation stress, strain and cyclic response rate of the VLAM are comparable to mammalian skeletal muscle. Because of the internal heating and cooling mechanism, VLAM shows very robust actuating performance within wide range of environmental temperature.   

Colloidal Micromachines Regulated by Liquid Crystals

Reconfigurable microdevices have become a subject of intense interest due to their ability to harvest energy and change shape on demand. These attributes allow them to be used as robotic structures, constituents for self-healing materials, and switchable metamaterials. Yet, many of these structures are limited in utility by lack of control over their dynamics. Accordingly, much work has been done to engineer their shape, composition, and actuation as a means to control dynamics; however, little is known about regulation of their dynamics in complex fluid milieu. Here, we show how actuation of microdevices made from the assembly of patchy magnetic microcubes, which we refer to as “microbots”, can be regulated by the anisotropic viscoelastic environment of a liquid crystal (LC). We show that the elastic energy arising from the strain of LC around microbots directly influences their folding dynamics, which can be tuned by tailoring: (i) the far-field orientation of the LC and (ii) the local ordering of the LC at the microbot surfaces. These findings represent a first step towards establishing a general set of design rules to control the dynamics of actuating devices via use of anisotropic fluids.   

Programming Complex and Arbitrary Shape Changes in Liquid Crystal Elasotmers

Liquid crystal elastomers (LCEs) are reversible shape responsive materials that are promising for many applications including biomedical devices, microfluidic pumps and soft robotics. To date, programming complex shape changes in LCEs requires locally aligning the liquid crystals throughout the material to form a predetermined director profile. Here we demonstrate a straight-forward method that does not rely on foresight of the liquid crystal director to program complex reversible shape changes. Using an optimized two-step synthesis method we mechanically deform the LCEs into arbitrary shapes between the first and second cure steps. The resulting competitive double-network LCE is capable of transitioning between its initial shape and its programmed shape when heated and cooled, respectively. We demonstrate the versatility of this method in a variety of shape changes including films capable of transforming into a flower and a face.   

Dynamically morphing microchannels in liquid crystal elastomer coatings with extended disclinations

Topological defects in liquid crystal elastomers (LCE) drive complex stimuli-responsive deformation. Previous studies examined short disclinations oriented parallel to the surface normal of a thin film or coating [1-3]. Here we examine an array of parallel extended disclinations oriented in-plane in an LCE coating on a rigid substrate, produced by forming the LCE between substrates with prescribed anchoring [4]. On heating, the coating morphs to form an array of parallel microchannels, each located above a disclination. To understand this shape evolution, we model formation of disclinations via numerical minimization of the Frank free energy, then use finite element simulation to calculate thermo-responsive deformation. Results are compared with experiments and with analytical calculations in the small-strain limit. We demonstrate use of thermo-responsive microchannels to perform particle sorting. Future uses may include applications in microfluidics and tissue engineering. [1] McConney et al, Adv Mater 25, 5880 (2013); [2] Babakhanova et al Nat Commun 9, 456 (2018); [3] Konya et al, Front Mater 3, 24 (2016); [4] Wang et al Nat Commun 8, 388 (2017).   

Electrorheological model based on liquid crystals membranes with applications to outer hair cells

Liquid crystal flexoelectric actuation uses an imposed electric field to create membrane bending, this phenomenon is found in Outer Hair Cells (OHC) located in the inner ear, whose role is to amplify sound through generation of mechanical power. Oscillations in the OHC membranes create periodic viscoelastic flows in the contacting fluid media. A key objective of this work on flexoelectric actuation relevant to OHC is to find the relations and impact of the electro-mechanical properties of the membrane, the rheological properties of the viscoelastic Jeffrey’s media, and the frequency response of the generated mechanical power output. When the inertia is neglected, the system follows a non-monotonically behavior in the power spectrum. This behavior is associated to the solvent contributions related to the retardation-Jeffreys mechanisms.   

Patterning order and disorder with an angle: Modeling dual-phase nematic elastomer ribbons

A wide variety of out-of-plane actuation behavior can be encoded in liquid crystalline polymer networks thru implementing complex microstructures in the nematic director field. While most imprinted designs are based on spatial variations of the nematic director, no theoretical studies have been made on dual-phase elastomers: samples that combine well-defined regions with nematic order and isotropic regions. Depending on patterning design, these materials exhibit a variety of actuation behavior, going from helical twisting to chiral bending and accordion folding. By implementing finite element elastodynamics simulations at the continuum level, we demonstrate this actuation variety based on several key design factors: director orientation, pattern orientation, as well as domain and sample size. Our simulations studies show an exceptional agreement with experimental observations, providing insights for further development of soft solids with complex hybrid microstructures.
  

Polymer nanocomposites with reversible heat stiffening properties

Inspired by the defense mechanism of sea cucumber, a stimuli-responsive nanocomposite was fabricated that can reversibly increase its stiffness upon exposure to warm water. Experimentally, polymers with lower critical solution temperature (LCST) were grafted on cellulose nanocrystals embedded into a viscoelastic matrix. This material shows reversible heat-stiffening behavior analogous to sea-cucumber dermis. The stiffening behavior was hypothesized to occur due to formation of a percolating network by the nanofillers above the transition temperature. Energy Conserving Dissipative Particle Dynamics (EDPD) simulations were performed to examine the hypothesis. According to experimental data and simulation results, grafted LCST polymers disrupt the interactions between the nanocrystals below the transition temperature and upon exposure to warm water collapse of the LCST chains enhances the interactions between the cellulose nanocrystals and results in the subsequent stiffening.
  

Dynamic Microcapsules from Complex Emulsion Drops

Microcapsules are widely employed to protect and release sensitive cargo at predetermined trigger-events and rates. Since common release mechanisms involve degradation or destruction of the protective shell, their functionality is one-directional with single-use applicability. We demonstrate the fabrication of microcapsules that reversibly respond to external stimuli by changing the shell membrane’s property without structural degradation, enabling the repeated and dynamic change of its permeability upon changing trigger events. The encapsulating shell in our system acts as an active gate-keeper, regulating diffusion in and out of the aqueous core compartment. I will describe microfluidic fabrication methods for complex emulsion drops and the development of a number of polymer chemistries that allow for the synthesis of trigger-responsive hydrogel shells directly around water drops without the need of sacrificial templates. The trigger responsive microcapsules are distinctly different from microgels, as the properties in microcapsules are dictated only by the state of the shell membrane that can make up less than 10% of the microparticle. The shape, size, and permeability of the microcapsules are dynamically and actively tunable with external triggers.   

Modelling discrete differential swelling in a bi-strip rolled gel

We derive an analytical model that allows us to quantitatively predict the features of 2D-to-3D shape changes in polymer gels that encompass different degrees of swelling within the material and thus, can model different regions of growth within the sample. Such gels can be realized, for example, by introducing variations in the cross-link density within the network or polymerizing the chains to be relatively longer in one area of the sample than another. Focusing on a bi-strip gel that swells into a “bi-roll”, we determine the radii and amplitudes within a given roll, and the length of the transition layer between the two rolls. The predictions from our model agree quantitatively with available experimental data. In addition, we carry out numerical simulations that account for the complete non-linear behavior of the gel, and show good agreement between the analytical predictions and the numerical results. Models that provide quantitative predictions on the final morphology in such heterogeneously swelling hydrogels are useful not only for understanding growth patterns in biology, but also for establishing how to accurately tailor the structure of gels to meet the requirements of various technological applications.