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We use field emission scanning electron microscope (FE-SEM) to investigatethe growth of palladium colloids over the surface of thin films of WO3/glass.The film is prepared by Pulsed Laser Deposition (PLD) at differenttemperatures. A PdCl2 (aq) droplet is injected on the surface and in thepresence of steam hydrogen the droplet is dried through a reduction reactionprocess. Two distinct aggregation regimes of palladium colloids are observedover the substrates. We argue that the change in aggregation dynamics emergeswhen the measured water drop Contact Angel (CA) for the WO3/glass thin filmspasses a certain threshold value, namely CA = 46 degrees, where a crossover inkinetic aggregation of palladium colloids occurs. Our results suggest that themass fractal dimension of palladium aggregates follows a power-law behavior.The fractal dimension (Df) in the fast aggregation regime, where the measuredCA values vary from 27 up to 46 degrees, according to different substratedeposition temperatures, is Df = 1.75 (0.02). This value of Df is in excellentagreement with kinetic aggregation of other colloidal systems in fastaggregation regime. Whereas for the slow aggregation regime, with CA = 58degrees, the fractal dimension changes abruptly to Df=1.92 (0.03). We have alsoused a modified Box-Counting method to calculate fractal dimension ofgray-level images and observe that the crossover at around CA = 46 degreesremains unchanged.

Observation of a crossover in kinetic aggregation of Palladium colloids