The rehydration properties of original whey protein isolate (WPI_C) powder and spray-dried WPI prepared from either unheated (WPI_UH) or nanoparticulated WPI solutions were investigated. Nanoparticulation of whey proteins was achieved by subjecting reconstituted WPI_C solutions (10% protein, w/w, pH 7.0) to heat treatment at 90 °C for 30 s with no added calcium (WPI_H) or with 2.5 mM added calcium (WPI_HCa). Powder surface nanostructure and elemental composition were investigated using atomic force microscopy and X-ray photoelectron spectroscopy, followed by dynamic visualisation of wetting and dissolution characteristics using environmental scanning electron microscopy. The surface of powder particles for both WPI_UH and WPI_C samples generally appeared smooth, while WPI_H and WPI_HCa displayed micro-wrinkles with more significant deposition of nitrogen and calcium elements. WPI_H and WPI_HCa exhibited lower wettability and solubility performance than WPI_UH and WPI_C during microscopic observation. This study demonstrated that heat-induced aggregation of whey proteins, in the presence or absence of added calcium, before drying increases aggregate size, alters the powder surface properties, consequently impairing their wetting characteristics. This study also developed a fundamental understanding of WPI powder obtained from nanoparticulated whey proteins, which could be applied for the development of functional whey-based ingredients in food formulations, such as nanospacers to modulate protein–protein interactions in dairy concentrates.