Silicone oil

The electro-rheological properties of the sulfonated PS particle dispersions were measured under direct current (DC) electric fields using a Physica MCR300 Rotational Rheometer equipped with a coaxial cylinder Physica ERD CC/27 (Malvern Instruments Ltd., Malvern, UK). Silicone oil (Dow Corning, Auburn, AL, USA) with a viscosity of 50 cSt, an electrical conductivity of approximately 3–5 pS·m⁻¹, and a relative permittivity of around 2.8, was used for the experiments. All the rheological measurements were performed at a constant temperature of 23 °C. The flow curves were collected in the shear rate range between 0.1 and 1000 s⁻¹, and at different electric field strengths, namely 0.5, 1, 2, 4, 6, and 8 kV/mm, respectively.

The experimental setup for the electric field-driven particle assembly of PS particles consisted of a signal electrode made of a stainless steel medical needle mounted on a mechanical x-y-z translational stage, a grounded conductive plate placed on another mechanical x-y-z translational stage, a CMOS camera (UI-3590CP-C-HQ, IDS Imaging Development Systems GmbH, Obersulm, Germany) mounted on a high-magnification zoom lens system (MVL12X3Z, Thorlabs, Inc., Newton, NJ, USA), a signal generator, a voltage amplifier, and a PC for recording images and movies. The high-voltage electric signal was obtained by amplifying a low-voltage signal (SDG1025, SIGLENT Technologies America, Inc., Solon, OH, USA) using a high-voltage bipolar amplifier (10HVA24-BP1 HVP High Voltage Products GmbH, Munich, Germany). The AC electric signal was always square-shaped and bipolar, and its value was provided as the root-mean-square (RMS) value. Silicone oil (200/350 mPa·s, Dow Corning, Auburn, AL, USA, electrical conductivity approximately 3–5 pS·m⁻¹, relative permittivity 2.8) droplets containing sulfonated PS particles were introduced onto the conductive plate. A needle-shaped electrode was dipped into the dispersion and, after the voltage was applied, the electrode was slowly lifted up to pull the PS particles out of the dispersion.

The PDMS brick is made of SYGRID 184 (Dow Corning Co., Shanghai, China) and cured at 85 °C for 4 h. Silicone oil (Dow Corning Co., Shanghai, China) with the viscosity of 20 mPa·s and 500 mPa·s is used as the inner fluid and outer fluid, respectively, while 5% (w/w) solution of Poly(vinyl alcohol) 1788 (Aladdin Biochemical Technology Co., Shanghai, China) was used as the middle fluid. Three syringe pumps (LSP02-1B, Longer Precision Pump Co., Baoding, China) are used to inject the fluids into the devices. The visualization system is comprised of a microscope (SZX7, Olympus, Tokyo, Japan), a high-speed camera (500K-M2, Photron, Tokyo, Japan), a computer, and a light source. All experiments are conducted at constant room temperature of 22 °C.