Strain stiffening and positive piezoconductive effect of liquid metal/elastomer soft composites

Liquid metal (LM) droplets have been demonstrated as an effective additive to transform elastomers into multifunctional composites that remain highly conductive under large mechanical deformation. Herein, we present a new phenomenon of strain hardening of polydimethylsiloxane (PDMS) composites with Galinstan LM droplet inclusions. Small LM inclusions increase the stiffness of PDMS matrix due to the surface tension effect at the liquid/solid interface after the droplet size becomes less than the elastocapillary length during stretching. The tensile strength was 0.60 MPa, 1.10 MPa and 1.72 MPa for PDMS composites with LM contents of 4.8 vol%, 7.5 vol% and 11.0 vol%, respectively, which represents 58%, 189%, 353% increase in comparison with the PDMS matrix. The electrical conductivity of the PDMS/LM composites with 4.8 vol% and 7.5 vol% LM droplets increased with the strain of the specimens, overcoming the shortcomings of rising resistance of conventional polymer/conductive filler composites under mechanical deformation. When subjected to tensile cyclic loading, the PDMS/LM composites demonstrated excellent durability after 2000 cycles with only 13% decrease (with 10% strain amplitude) in relative resistance for the composite with 7.5 vol% LM droplets and 7% reduction (with 20% strain amplitude) for that with 11.0 vol% of LM. The hybridization of carbon nanofibres (CNFs) however affected the distribution of LM droplets in the matrix and only showed enhanced mechanical property (a 52% increase in tensile strength and 31% increase in Young's modulus in PDMS4.8LM0.1CNF). This new class of soft materials create new opportunities of designing wearable electronics, soft robotics, stretchable heater and biomedical implants.