Research into evaporating droplets on patterned surfaces has grown exponentially, since the capacity to control droplet morphology has proven to have significant technological utility in emerging areas of fundamental research and industrial applications. Here, we incorporate two interest domains-complex wetting patterns of droplets on structured surfaces and the ubiquitous coffee-ring phenomenon of nanofluids containing dispersed aluminium oxide particles. We lay out the surface design criteria by quantifying the effect of pillar density and shape on the wetting footprint of droplets, yielding complex polygon droplet geometries. Our work is not constrained to pure liquids only, as we delve into the shape selection of particle-laden droplets of different concentrations. We visualise the deposition patterns through microscopy on surfaces exhibiting different features and further establish the ordering of particles on microscale surface asperities. At a high nanofluid concentration, we observe intriguing self-assembly of particles into highly ordered intricate structures. The collective findings of this work have the potential to enhance many industrial technologies, particularly attractive for high performance optical and electrical devices.

A study was undertaken of ethanol droplet evaporation on structured surfaces of pillars (square pillars of variable dimensions and spacing of order microns, and cylindrical of various spacings). On seasoned perflourodecyltrichlorosilane surfaces, significant films were observed extending far beyond the initial contact line for pure ethanol droplets, most prominently for 4 microlitre droplets. On parylene coated surfaces, similar imbibed films were seen for 1.5 microlitre droplets of 50% by volume ethanol-water mixture. This film acts as an additional surface for evaporation and it appears that the droplet then serves as a reservoir for feeding the film until the evaporation process is completed, rather than evaporation being governed by evaporation at the contact line. The droplets with films show higher evaporation rates.

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