| dc.description.abstract | Understanding 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. | en_US |
