Fe3o4 nanoparticles

N-Isopropylacrylamide (NIPAAm) (700 mM) monomer and N,N’-Methylenebis (acrylamide) (BIS) (8.6 mM) crosslinker were dissolved in deionized water. Fe₃O₄ (2.5 wt% of monomer solution) was prepared in water and blended with the monomer solution. The mixture was stirred for 15 min to obtain a homogeneous dispersion of the magnetite nanoparticles. The radical initiator Ammonium Persulfate (APS) (0.1 wt% of mixture) and accelerator Tetramethylethylenediamine (TEMED) (2.25 wt% of mixture) were then added and mixed to initiate the redox reaction. After mixing, the solution was injected into a cylindrical mold that was 400 μm in diameter and allowed to stand overnight at room temperature to achieve the desired cylindrical iron-encapsulated PNIPAAm hydrogel particles. In order to prevent the particle from sticking to the substrates during experiment, we formed an intercalating lubricating layer by immersing the particles in Tween 20 solution until fully swollen^{21 (link)}. The diameters of hydrogel particles were 700 μm at the swollen state.
The chemicals, including NIPAAm monomer, BIS cross-linker, TEMED, APS were obtained from Sigma Aldrich (St. Louis, MI). The magnetic Fe₃O₄ nanoparticles (15–30 nm diameter) were obtained from US Research Nanomaterials, Inc (Houston, TX).

PETA (technical grade), BPADA (97%), 4-hydroxyanisole (MEHQ, 99%), and isopropanol (IPA, 99%) were purchased from Sigma Aldrich. Fe₃O₄ nanoparticles were purchased from US Research Nanomaterials Inc. PGMEA (99%) was purchased from Dieckmann. Photoresist IP-Dip™ was purchased from Nanoscribe. All chemicals/photoresists were used without further purification.

Looking at literature examined [50 (link)] and our previous experiences [29 (link)], it was found that Fe₃O₄-water nanofluid would work best in the present experiment. The experiment was conducted using 99.5% pure Fe₃O₄ nanoparticles purchased from US Research Nanomaterials, Inc, Houston, TX, USA. The particles were listed as having spherical morphology and an average particle size of 15 to 20 nm. The nanofluid was formulated by mixing the Fe₃O₄ nanoparticles in deionized water with 1.0% concentration by weight, before it was sonicated for thirty minutes to ensure the suspension was well-dispersed. The necessary physical properties of the nanofluid, namely density, specific heat, and volume fraction, were calculated using Equations (1) through (3), as given by Sundar et al. [15 (link)]: $${\rho }_{nf}=\varphi {\rho }_p+\left(1-\varphi \right){\rho }_{bf}$$
$$c_{nf}=\frac{\varphi \left(\rho c\right)+\left(1-\varphi \right){\left(\rho c\right)}_{bf}}{{\rho }_{nf}}$$
$$\varphi =\frac{w{\rho }_{bf}}{{\rho }_p\left(1-w\right)+w{\rho }_{bf}}$$
where $\rho$ is the density in kg/m³, $\varphi$ is the nanoparticle volume concentration, $c$ is the specific heat in J/kg·K, and $w$ is the pure weight fraction of the nanofluid. Parameter nomenclature is reviewed in detail in Table A1 of Appendix A. Additionally, in Table 1 below, the important properties of the nanoparticles, base fluid, and nanofluid are specified. These values were determined by Sundar et al. [15 (link)] for various temperatures and were used to interpolate for the relevant temperature.