Meie Li

Modeling programmable deformation of self-folding all-polymer structures with temperature-sensitive hydrogels

Combination of soft active hydrogels with hard passive polymers gives rise to all-polymer composites. The hydrogel is sensitive to external stimuli while the passive polymer is inert. Utilizing the different behaviors of two materials subject to environmental variation, for example temperature, results in self-folding soft machines. We report our efforts to model the programmable deformation of self-folding structures with temperature-sensitive hydrogels. The self-folding structures are realized either by constructing a bilayer structure or by incorporating hydrogels as hinges. The methodology and the results may aid the design, control and fabrication of 3D complex structures from 2D simple configurations through self-assembly. (Some figures may appear in colour only in the online journal)

Modeling deformation and contacts of pH sensitive hydrogels for microfluidic flow control

Following a recently developed theory for the constrained swelling of pH-sensitive hydrogels and the finite element user-subroutine technique, we modeled deformation and contacts of pH-sensitive hydrogels for real purpose microfluidic flow control systems. A jacket valve consisting of three cylindrical hydrogels coated on three fixed pillars and a hybrid hydrogel/PDMS system are modeled and analyzed in this paper. We present the deformation and multiple contacts of hydrogel based flow control systems when the pH of the external solution is switched to various values. Also included is the pressure in the contacts, stress distribution, sensitivity of swelling ratio with respect to pH values, and in addition the influence of initial fabrication imperfections.

Predicting origami-inspired programmable self-folding of hydrogel trilayers

Imitating origami principles in active or programmable materials opens the door for development of origami-inspired self-folding structures for not only aesthetic but also functional purposes. A variety of programmable materials enabled self-folding structures have been demonstrated across various fields and scales. These folding structures have finite thickness and the mechanical properties of the active materials dictate the folding process. Yet formalizing the use of origami rules for use in computer modeling has been challenging, owing to the zero-thickness theory and the exclusion of mechanical properties in current models. Here, we describe a physics-based finite element simulation scheme to predict programmable self-folding of temperature-sensitive hydrogel trilayers. Patterning crease and assigning mountain or valley folds are highlighted for complex origami such as folding of the Randletts flapping bird and the crane. Our efforts enhance the understanding and facilitate the design of origami-inspired self-folding structures, broadening the realization and application of reconfigurable structures.

Stress evolution in a phase-separating polymeric gel

A polymer network can swell tremendously to form a gel which is typically transparent at room temperature. Upon temperature quenching, however, the gel can undergo phase separation and become opaque. We revisit and formulate the dynamics of phase separation of gels through co-evolution of polymer volume fraction and left Cauchy–Green tensor; both are physical and measurable quantities. A hybrid Fourier spectral method and an isotropic finite difference method is proposed to solve the evolution equations, and the scheme is verified to be efficient for either an isotropically or anisotropically swollen gel. For the isotropic swelling gel, a percolating network structure, where the shrunken phase encloses the solvent-rich phase, is formed during phase separation. With the formation of network structure, an inhomogeneous stress field builds up within the network and evolves simultaneously with concentration modulation. The effective stress levels in the common vertices of several shrunken phases are relatively low while the network segments between two vertices constitute the high stress region. A plausible stress-supporting mechanism is proposed to explain the formation of network structure and the phase-inversion phenomenon.

Photo-thermo-mechanically actuated bending and snapping kinetics of liquid crystal elastomer cantilever

A composite of liquid crystal elastomer (LCE) incorporated with carbon nanotubes (CNTs) can convert absorbed photon energy into thermal energy to trigger the phase transition of the LCE, resulting in photo-thermo-mechanically actuated devices. We model the transient temperature distribution and the bending kinetics of a straight cantilever beam actuator under the radiation of a laser diode (LD) light. Three possible bending modes of the beam for various LD light powers are identified. The temperature distribution and the bending modes are found to be in good agreement with the reported experimental observations. The underlying deformation mechanisms and bending modes are manifested by probing the stress evolution and propagation of nonzero stress regions during the bending process. For a beam that is initially slightly curved, we also predict the possibility of snap-through instability, and three typical phases of snapping are captured. This procedure paves the way for the design of LCE-based soft actuators.

Instability of liquid crystal elastomers

Nematic liquid crystal elastomers (LCEs) contract in the director direction but expand in other directions, perpendicular to the director, when heated. If the expansion of an LCE is constrained, compressive stress builds up in the LCE, and it wrinkles or buckles to release the stored elastic energy. Although the instability of soft materials is ubiquitous, the mechanism and programmable modulation of LCE instability has not yet been fully explored. We describe a finite element method (FEM) scheme to model the inhomogeneous deformation and instability of LCEs. A constrained LCE beam working as a valve for microfluidic flow, and a piece of LCE laminated with a nanoscale poly(styrene) (PS) film are analyzed in detail. The former uses the buckling of the LCE beam to occlude the microfluidic channel, while the latter utilizes wrinkling or buckling to measure the mechanical properties of hard film or to realize self-folding. Through rigorous instability analysis, we predict the critical conditions for the onset of instability, the wavelength and amplitude evolution of instability, and the instability patterns. The FEM results are found to correlate well with analytical results and reported experiments. These efforts shed light on the understanding and exploitation of the instabilities of LCEs.

SURFACE INSTABILITY OF A SWOLLEN CYLINDER HYDROGEL

Cylinder hydrogel is simple in geometry and easy to synthesize, therefore was widely used to investigate the swelling/shrinking instability of hydrogel and many instability patterns were accumulated in the literature. The mechanism of instability pattern formation of this unique configuration, nevertheless, is far from being fully understood. We applied and extended the recently developed nonlinear theory of polymer gels into cylindrical coordinates, and performed linear perturbation analysis of swelling-induced stability of a constrained cylinder hydrogel. We derived the incremental formulations of stresses and the associated equilibrium equations. We obtained the critical conditions for the onset of instability and probed in details the effects of various parameters on the stability diagram of the hydrogel. The physical meaning of the variation of stability diagram was also interpreted.

Modeling Contacts of Ionic Polymer Metal Composites Based Tactile Sensors

We report the first attempt to model the contacts of an ionic polymer metal composite (IPMC) based tactile sensor. The tactile sensor comprises an IPMC actuator, an IPMC sensor and the target to be detected. The system makes use of multiple contacts to work: the actuator comes into contact with the sensor and pushes the movement of sensor; the contact between the sensor and the object detects the existence and the stiffness of the target. We integrate modeling of various physical processes involved in IPMC devices to form a simulation scheme. An iteration and optimization strategy is also described to correlate the experimental and simulation results of an IPMC bending actuator to identify the two key parameters used in electromechanical transduction. Modeling the multiple contacts will aid the design and optimization of such IPMC based soft robotics.

Finite element implementation of poroelasticity theory for swelling dynamics of hydrogels

Hydrogel can swell to many times of its dry volume, resulting in large deformation which is vital for its function. The swelling process is regulated by many physical and chemical mechanisms, and can, to some extent, be fairly described by the poroelasticity theory. Implementation of the poroelasticity theory in the framework of finite element method would aid the design and optimization of hydrogel-based soft devices. Choosing chemical potential and displacement as two field variables, we present the implementation of poroelasticity tailored for hydrogel swelling dynamics, detail the normalization of physical parameters and the treatment of boundary conditions. Several examples are presented to demonstrate the feasibility and correctness of the proposed strategy.

Growth morphologies of a binary alloy with low anisotropy in directional solidification

Two new classes of growth morphologies, called doublons and seaweed, were simulated using a phase-field method. The evolution of doublon and seaweed morphologies was obtained in directional solidification. The influence of orientation and velocity on the growth morphology was investigated. It was indicated that doublons preferred growing with its crystallographic axis aligned with the heat flow direction. Seaweed, on the other hand, could be obtained by tilting the crystalline axis to 45°. Stable doublons could only exist in a range of velocity regime. Beyond this regime the patterns formed would be unstable. The simulation results agreed with the reported experimental results qualitatively.