In this study, the evaporation characteristics and drying patterns of various sessile ferrofluid droplets on certain substrate under horizontal magnetic fields of controlled intensities are reported. The effects of droplet concentration and magnetic field intensity on the duration of each evaporation stage and drying patterns of droplets have been systematically investigated. It turned out that a plateau appears at the initial stage of evaporation in the absence of magnetic field and it was found that the plateau value is positively correlated with the concentration of ferrofluid droplets. Under the external magnetic field, the evaporation time of droplets decreases, the stage of contact line retreat extends, the stage of late pinning mode shortens, and the deposition area of ferrofluid droplet decreases compared to that of without magnetics field. The deposition area increases gradually and becomes more uniform with the increase of magnetic field. The decrease of friction force which is due to the decrease of the number of nanoparticles at the contact line under external magnetic field is the main reason for the observed phenomena. We found that the coffee ring and the uniform deposition inside the droplet will be destroyed when the magnetic field intensity is higher than a critical value. Our work has a significant reference value for the evaporation of sessile magnetic fluid droplets under the applied magnetic field, especially when the drying pattern needs to be precisely controlled, such as in spray or biomedicine.

The sol-gel state of smectite clay dispersions varies with the volume fraction of clay and electrolyte concentration. In this study, it was elucidated that the drying patterns of droplets from four types of smectite clay dispersions vary according to their sol-gel states. Droplets in the sol state exhibited a ring-shaped pattern, while those in the gel state showed a bump-shaped pattern. Near the boundary between the sol and gel states, patterns featuring both ring and bump structures were observed regardless of whether the droplets were on the sol or gel side. When guest particles or molecules were introduced into the clay dispersion, they dispersed uniformly within the system, and the drying pattern depended on the sol-gel state of the droplets. These findings suggest that the presence or absence of convection within the droplets during drying governs the drying pattern.

Drying kinetics and the moisture distribution map of radish slabs under different drying methods (hot-air drying (HAD), microwave drying (MD), and hot-air and microwave combination drying (HMCD)) were determined and visualized by hyperspectral image (HSI) processing coupled with a partial least square regression (PLSR)-variable importance in projection (VIP) model, respectively. Page model was the most suitable in describing the experimental moisture loss data of radish slabs regardless of the drying method. Dielectric properties (DP, ε ) of radish slices decreased with the decrease in moisture content (MC) during MD, and the penetration depth of microwaves in radish was between 0.81 and 1.15 cm. The PLSR-VIP model developed with 38 optimal variables could result in the high prediction accuracies for both the calibration ( R c a l 2 = 0.967 and RMSEC = 4.32 % ) and validation ( R v a l 2 = 0.962 and RMSEC = 4.45 % ). In visualized drying patterns, the radish slabs dried by HAD had a higher moisture content at the center than at the edges; however, the samples dried by MD contained higher moisture content at the edges. The nearly uniform drying pattern of radish slabs under HMCD was observed in hyperspectral images. Drying uniformity of radish slabs could be improved by the combination drying method, which significantly reduces drying time.

Drying dissipative patterns of the water-soluble gelatin sheet wetted by an aqueous droplet were observed as a function of time elapsed. The arrayed clusters of dishes formed at the broad ring area and grew outward toward multiple arrays with time. The drying patterns formed by the cooperative contribution of wetting, swelling, dissolving, evaporative, convectional, sedimentary and solidifying processes. Drying patterns were studied also for aqueous ethanol, aqueous NaCl solutions and dispersions of colloidal silica and poly(methyl methacrylate) spheres. The dish patterns were observed for the water-soluble substrates, for the first time, in this work.