Nanoparticle deposition pattern during colloidal droplet evaporation as in-situ investigated by Low-Field NMR: The critical role of bound water

Abstract

The evaporation of a colloidal droplet on a solid surface produces a ring-like structure at the pinned contact line due to induced capillary flow, also known as the coffee-ring effect (CRE). However, the addition of certain additives can significantly reduce CRE, whose physical origin remains in debate. In this study, the time-resolved low field (LF)-NMR technique highlights the effect of different water fractions in colloidal droplets on the ultimate distribution of silica nanoparticles during evaporation. With the assistance of ¹H T₂ relaxometry, the impact of decreasing evaporation rate (J) and additives on the fractions of bound, trapped, and free water can be obtained. Utilizing the T₂ Carr-Purcell-Meiboom-Gill (CPMG) sequence approach, in-situ tracking during droplets evaporation, with varying J, was obtained to conclude the minimum existence time of bound water required during evaporation for CRE suppression. As the droplet J decreases, the competition between the time scale of totally droplet evaporation t_F and the time spent of bound water during evaporation t_B may influence the ring formation. Experimentally a shorter duration of t_B/t_F ≤ 0.5 is required for formating the coffee ring structure and successfully suppressed when its existence time surpasses a particular threshold ≥ 0.5.

Keywords: Coffee-ring effect (CRE); Low-density polyethylene (LDPE); Surfactant; T(2) Carr-Purcell-Meiboom-Gill sequence (CPMG); Time-domain LF-NMR.