Magnetic particle spectroscopy analysis of the coffee ring effect in sessile ferrofluid droplets.

HYPOTHESIS: Superparamagnetic iron oxide nanoparticles (SPIONs) can be employed as magnetoactive agents to probe consolidation phenomena owing to the sensitivity of their magnetization response to interparticle distance through dipolar interactions. We hypothesize that this property could be used in the analysis of the coffee ring effect, which refers to an evaporation-induced capillary flow-driven build-up of non-volatile solutes near the three-phase contact line of sessile droplets, important for many applications. EXPERIMENTS: Magnetization response of SPION ferrofluid droplets was measured with magnetic particle spectroscopy (MPS) operated in the surface sensor configuration. The results were compared to in-situ microscopy observations and theoretical predictions of non-interacting SPIONs using the Langevin equilibrium magnetization model while accounting for the droplet shape evolution with the diffusion-limited evaporation model. FINDINGS: The results demonstrate that the surface sensor MPS is sensitive to the magnetization response changes induced by the formation of the coffee ring deposit. Excellent qualitative agreement was found between the spectral magnetic moments and their ratios measured with MPS and theoretical predictions in dilute systems when interparticle dipolar interactions are considered negligible. Deviations from theoretical predictions and the emergence of a phase lag were observed at late drying stages attributed to increased dipolar interactions in the crowded coffee ring deposit. Our study highlights the potential of utilizing MPS in the analysis of drying related processes involving magnetoactive components.