Rheological Memory and Thixotropic Behavior of a Novel Water‐Based Magnetic Thixotropic Fluid for Drop‐On‐Demand 3D Printing

In this study, etched Fe 3 O 4 magnetic nanoparticles were prepared using a chemical coprecipitation method combined with a self‐forming acid etching technique. Through characterization by transmission electron microscopy, X‐ray diffraction, vibrating sample magnetometer, and Zeta potential analysis, it was confirmed that the particles had an average particle size of 9.5 nm. After etching, the particle size and inverse spinel crystal structure remained unchanged, and the surface of the particles acquired positive charge while exhibiting superparamagnetism. The optimal etching conditions were found to be a concentration of 0.4 mol/L, an etching amount of 6%, and an etching time of 10 min. Based on the positive–negative charge adsorption mechanism between DE bentonite and the etched Fe 3 O 4 magnetic nanoparticles, a water‐based magnetic thixotropic fluid (MTF) was successfully prepared. This material demonstrated significant rheological memory, and its stability was verified through L–C oscillation circuit tests. When the mass ratio of etched magnetic nanoparticles to DE bentonite was 40:3, the material achieved optimal performance with a saturation magnetization of 17.5 emu/g. Rheological tests, including shear flow behavior, dynamic viscoelasticity, and modulus recovery, revealed the microstructural evolution of the material under both magnetized and nonmagnetized conditions. The results demonstrated shear‐thinning behavior, secondary yield behavior, sol–gel‐like transition, and rheological memory characteristics, proving its ability to undergo liquid–solid transformation. These unique properties render the water‐based MTF promising applications in fields such as 3D printing, smart control, and magnetic‐responsive composites, especially in scenarios requiring precise control over rheological and magnetic responsiveness.