Ordered stripes to crack patterns in dried particulates of DNA-coated gold colloids via modulating...

The surface pattern in dried droplets of nanoparticle suspension possesses direct correlation with the evaporation profile, which apart from the bulk parameters, can also be altered by tuning the nanoscale interactions. Here, we show that, for sessile drops of DNA-coated gold nanoparticle (DNA-AuNP) solution, the alteration in evaporation pathway of TPCL (three-phase contact line) from stick-slip to mixed mode leads to a surface morphological transition from concentric rings with stripes to radial crack formation within the coffee ring deposit. A freshly cleaned silicon substrate offers hydrophilic/favorable substrate-nanoparticle interaction and produces multiple ordered stripes due to stick-slip motion of the TPCL. Using a SiO₂/Si substrate with ∼200 nm of oxide layer leads to an increase in the initial water contact angle θ_i-w by ∼40°, due to increased hydrophobicity of the substrate. Three distinct modes of evaporation are observed - constant contact radius (CCR), constant contact angle (CCA) and mixed mode, resulting in the formation of radial cracks on a thick coffee ring structure. The critical thickness (h_c), beyond which the cracks start to appear, was measured to be ∼600 nm and is in close agreement with the theoretical estimate of ∼510 nm. Through in situ contact angle and ex situ SEM measurements, we provide an understanding of the observed surface morphological transition in the dried particulate at various nanoparticle densities. Further analysis of the coffee ring width (d), linear crack density (σ) and crack spacing (λ) provides insight into the mechanism of crack formation for droplets dried on oxide-coated substrates.