Size-dependent colloidal deposits of impacting droplets on an incline: role of impact speed and inclination angle*

We experimentally studied the dried deposits emanating from the impact of colloidal microspherical particle-laden aqueous droplets on surfaces inclined at angles α ∈[0 − 60^◦], with respect to the horizontal plane at Weber numbers (We) varied in the range of [0 - 46]. Elucidation of the variation in the deposit patterns as a function of particle size (diameter ∈[1.1 − 6.0] μm) and surface inclination angle are in focus. Quantifications utilized high-speed imaging and optical profilometry tools. On horizontal surfaces, typical “Coffee Ring” deposits in the contact line region were seen at a low We. On the inclined surfaces, asymmetric deposits along the advancing and receding fronts emerge, stemming from the combined action of capillary flow, curvature-driven advection, and sedimentation flow. Impact at higher We tends to produce elongated and dispersed deposits with enhanced asymmetry. We conducted a scaling analysis incorporating the balance between inertial and capillary forces for maximum spreading corroborates the observations. We distinctively introduced the concept of the ratio between the capillary and sedimentation flow strengths and elucidated the observations by scaling analyses. Overall, the distinct experimentation, along with the gained insights, would potentially advance various applications, including inkjet printing, coatings, and forensics.