Pulsed laser ablation for enhanced liquid spreading

Nanosecond pulsed laser ablation is shown to be an effective method for enhancing liquid spreading on surfaces where the intrinsic droplet static contact angle is less than 60 degrees. A simple theoretical model is developed to optimise the laser scanning strategy and maximise the resulting Wenzel roughness factor, r. Laser ablation experiments are then performed with a nanosecond pulsed fibre laser with emission wavelength of 1064 nm on stainless steel and aluminium alloy surfaces, with beam scanning based on model outcomes to achieve discrete ablation craters in the longitudinal and lateral directions. Adhesive, oil and ethanol droplets are deposited on untreated and laser-textured surfaces, with liquid spreading quantified via photography and image processing. Large increases in adhesive and oil droplet spreading area on laser-textured surfaces are accounted for in terms of a transition from Wenzel state to film regime, where liquid spreading is driven by a negative change in interfacial energy with progression of the imbibition front. By increasing r, laser ablation is found to increase the threshold contact angle below which transition to a film regime takes place. Further increases in r are found to further improve liquid spreading due to a larger negative interfacial energy gradient until the ablated volume becomes significant and liquid retention within the surface limits further gains. Eighteen- and seven-fold increases in spreading area are achieved for adhesive and oil, respectively, while no improvements are observed for ethanol due to immediate onset of a film state on untreated samples.