Control of Droplet Evaporation Contact Line Dynamics by Silicone Oil Grafting*

Understanding the contact line dynamics during droplet evaporation is of importance to many everyday and industrial applications. To date, most of the research on hydrophobic coatings focuses on the development of low surface energy promoters leading to high contact angles and low contact angle hysteresis (CAH). While less effort has been placed on understanding the optimisation of fast and easy fabrication parameters for the control and tuning of such droplet-surface interactions. In this work, we address how different silicone oil grafting parameters such as viscosity of the oil (5 cSt, 20 cSt, and 100 cSt), deposition method (pipette or dip coating), number of layers (1, 2 3, 5), etc. can enable the control of the contact line dynamics during droplet evaporation. Contact angles as high as 108° with contact angle hysteresis ranging from 1° to 20° are found after grafting one single layer of oil on a smooth silicon substrate. Moreover, depending on the fabrication procedure, the duration of different evaporation modes, i.e., constant contact angle (CCA), constant contact radius (CCR), and mixed modes, can be controlled. In addition, stick-slip behaviour is also noticed during evaporation, where the magnitude of the jumps and changes in contact angle can also be tuned by the silicone oil grafting procedure adopted. The findings reported here provide the relevant guidelines for the control of the droplet-surface interactions by simple and scalable functionalisation of a smooth intrinsically hydrophilic substrate with potential applications in ink-jet printing, crop dusting, heat exchangers, and biomedical applications, amongst others.