Enhanced Variable Stiffness and Variable Stretchability Enabled by Phase‐Changing Particulate Additives

Abstract

We introduce a novel phase-changing particulate that amplifies a composite's modulus change in response to thermal stimulus. This particulate additive consists of a low melting point alloy (Field's Metal; FM) formed into microparticles using a facile fabrication method, which enables its incorporation into polymer matrices using simple composite manufacturing processes. The effect of the solid-liquid phase change of the FM particles is demonstrated in two host materials: a thermally responsive epoxy and a silicone elastomer. In the epoxy matrix, this thermal response manifests as an amplified change in flexural modulus when heated, which is highly desirable for stiffness-changing move-and-hold applications. In the silicone matrix, the stretchability can be switched depending on the phase of the FM particles. This phenomenon allows the silicone to stretch and hold a strained configuration, and gives rise to mechanically programmable anisotropy as the FM inclusions are reshaped. FM particles present many opportunities where on-demand tunable modulus is required, and is particularly relevant to soft robotics. Because the melting temperature of FM is relatively close to room temperature, triggering the phase change, and thereby modulating the modulus, can be accomplished with low power consumption. We demonstrate the utility of these FM particle-containing composites as variable stiffness and variable stretchability elements targeting applications in the field of soft robotics.

Liquid metal nanocomposites

“…14,52,53 The supercooling effect limits the application of LM nanodroplets in composites with stiffness tuning capability as solid-liquid phase transition occurs at extremely low temperature. Unlike LM nanodroplets, phase transition of micro-/macroscale LM droplets [54][55][56][57] and uidic channels [58][59][60] have been be used for rigidity tuning and shape morphing with potential applications in so robotics. 6,61 In recent years, LM nanodroplets have been used as stretchable conductors in emerging electronics.…”

Temperature and rate dependent constitutive behaviors of low melt Field ’s metal

“…Low melting point metals (LMPM) [1] have been employed to fabricate functional materials and structures [2][3][4][5][6][7][8][9][10][11] with tunable stiffness, shape memory effect, self-healing behaviors, good thermal/electrical conductivity, etc. The most commonly used lead-free LMPMs include gallium (melting point Tm = 30℃) and Field's metal (Tm = 62℃), which are also called liquid metals sometimes [1].…”

“…This phase transition is accompanied with a loss of stiffness and the fusion of defects due to the melting process. Such characteristics can be employed to design and fabricate LMPM composites with tunable stiffness and self-healing behaviors [2][3][4]7,9]. Besides, a shape memory effect is attained if the LMPM composites are hot/cold programmed and re-melted [2,3,5,6,9,10], similar to the behavior of shape memory polymers.…”

“…This work has led to a surge of multifunctional materials based on LMPMs. For example, composites comprising of Field's metal particles and polymers have been synthesized and tested for their shape memory effect [5,9], stiffness-tuning ability [3], toughness enhancement [15], and self-healing behavior [9]. Moreover, researchers also studied the mechanical characteristics of gallium-based composites [6,10,16], which are similar to that of Field's metal composites.…”

Robust Three‐Component Elastomer–Particle–Fiber Composites with Tunable Properties for Soft Robotics

“…[42] Smart materials with tunable stiffness have been used as actuation mechanisms for soft grippers based on mechanical interlocking or dynamically tunable dry adhesion, [43,44] for soft crawling robots, [45,46] for minimal invasive continuum devices, [47] and for reconfigurable surfaces in drones. [28] However, the existing smart materials still exhibit limited tunability of mechanical stiffness and/or electrical/thermal conductivity (either too high or too low), and poor mechanical robustness for repeated use, [15][16][17][18]20,22,23,31] thus presenting challenges in terms of activation voltage (too high) and activation time (too long) for their applications in soft and reconfigurable robotics. [15][16][17][18]22,27,28] These limitations can be attributed to the fact that most of the existing smart composites are composed of one matrix component and one functional component with low aspect ratio.…”