Droplet impact and contact angle dynamics: From spreading and splashing to 3D printing.

PhD ThesisUnderstanding the contact line dynamics during droplet impact is critically important for industrial processes such as coating, the spraying of pesticides and for the design of anti-icing surfaces or impermeable clothing. It is known that, upon impacting on a solid, a droplet can spread, bounce off the substrate or splash depending on the liquid characteristics, the solid properties, the impact speed and the ambient pressure. In this thesis, we explore droplet impact in terms of some of these variables. Consequently, this work is focused on the experimental study of the contact line dynamics of impacting droplets on substrates ranging from wettable to non-wettable and from porous to non-porous. In particular, we focus on the parameters affecting: i) the splashing threshold of impacting droplets on solid substrates and ii) the penetration of impacting droplets through textiles. Furthermore, we apply our findings to the development of a liquid latex droplet-on-demand printing system. Most of the experiments in this thesis consist of the visualisation, by high speed imaging, of the impact of ethanol, water and aqueous glycerol droplets on solid and textile substrates. In addition, we present a custom-made Matlab algorithm that uses a polynomial fitting approach to extract the dynamic contact angle as a function of the contact line velocity. Moreover, we analyse the effect of droplet shape, the order of the fitting polynomial and the fitting domain, on the measurement of the contact angle on various stages following droplet impact. We use our experimental setup to demonstrate the importance of wettability and substrate roughness on the contact line dynamics and the impacting outcome. For smooth surfaces, we show that the maximum advancing contact angle ( max) is greater than 87 degrees for all the liquid/substrates. Moreover, we show that splashing depends on the substrate wettability and its threshold can be parameterised by max and the splashing ratio. Correspondingly, for rough surfaces, we determine that max increases with increasing substrate roughness. Furthermore, we establish that the ratio of the peak to peak roughness to the surface feature mean width, in conjunction with max and the splashing ratio, adequately predict the splashing threshold. Similarly, for the droplet impact dynamics on textiles, we find that the textile characteristics, such as the pore size and solid fraction, are critical for the impact outcome. Correspondingly, we find three different impact regimes, namely, ‘no penetration’, ‘capture’ and ‘complete penetration’. Additionally, by balancing the kinematic pressure with the capillary pressure, we find a critical pore size for the transition from capture to complete droplet penetration in terms of the Weber number. Finally, we present a setup that permits the printing, by droplet impact, of liquid latex on paper with a high solid content (60 wt %). The process is controllable and reliable, making the printing of patterns possible. With this setup, multilayer objects were created from pure liquid latex, as well as from micronized rubber powder and latex suspensions. These results demonstrate the potential of droplet-based additive manufacturing processes to produce prints of liquid latex and tire rubber reuse

Microfluidic jet impact: spreading, splashing, soft substrate deformation and injection

Hypothesis: Needle-free injections using microfluidic jets could be optimized by reducing splashing and controlling injection depth. However, this is impeded by an incomplete understanding on how jet characteristics influence impact outcome. We hypothesise that exploring the relation between microfluidic jet characteristics and substrate shear modulus on impact behavior will assist in predicting and giving insights on the impact outcome on skin and injection endpoints. Experiments: To do so, a setup using microfluidic chips, at varying laser powers and stand-off distances, was used to create thermocavitation generated microfluidic jets with ranging characteristics (velocity: 7–77 m/s, diameter: 35–120 μm, Weber-number: 40–4000), which were impacted on substrates with different shear modulus. Findings: Seven impact regimes were found, depending on jet Weber-number and substrate shear modulus, and we identified three thresholds: i) spreading/splashing threshold, ii) dimple formation threshold, and iii) plastic/elastic deformation threshold. The regimes show similarity to skin impact, although the opacity of skin complicated determining the threshold values. Additionally, we found that jet velocity has a higher predictive value for injection depth compared to the Weber-number, and consequently, the jet-diameter. Our findings provide fundamental knowledge on the interaction between microfluidic jets and substrates, and are relevant for optimizing needle-free injections