Page 203 - IJB-9-5

International Journal of Bioprinting Using droplet jetting for bioprinting

Figure 3. Droplet–substrate interaction classified based on the penetrative and nonpenetrative impact. The penetrative impact on non-Newtonian
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[57]
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materials such as PEGDA and gelatin ; penetrative impact on Newtonian material such as cell media and water . Nonpenetrative impact on
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[59]
viscoelastic substrate such as PDMS , and rigid materials such as glass slides and plasticware . Figures are reproduced under the terms of the Creative
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Commons Attribution 4.0 International License.
direction of forces. The surface flow is driven inward, on solid surfaces through the formation of jet based on
and the droplet is injected deep into the pool when the the impact . The transition between the coalescence and
[65]
droplet’s surface tension is greater than that of the pool. splashing can be predicted using a combination of Weber
2
On the other hand, the droplet spreads across the pool number and Froude number, Fr = VgD . The transition from
σ
surface when the surface tension is lower. The droplet may coalescence to short thick jet occurs when We = 34.7 Fr 0.145[67] .
penetrate the pool if it has the same surface tension as the The lower limit for large bubble entrapment zone with
water. Inhibiting direct contact and coalescence between short thin jet is We = 41.3 Fr 0.179[68] . This shift into the
droplet and receiving substrate results in droplet bouncing small bubble entrapment zone with long thin jet occurs at
on miscible/same liquid . We = 48.3 Fr 0.247 . Upper limit for small bubble entrapment
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Droplet impact on another liquid surface may result in is We = 63.1 Fr 0.257 , in which splashing with long thick jet
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floating, bouncing, coalescence, or splashing, as described happens without bubbles .
in a previous study and demonstrated in Figure 4 . A The velocity at which a droplet penetrates into the liquid
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floating drop will usually disappear after a few seconds is weakly dependent on the liquid’s viscosity . This is due
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into the liquid. Bouncing of the droplet can occur when to the air film that exists between the droplet and the liquid,
the impact velocity and the size of a single droplet is small which primarily serves to cushion droplet penetration into
enough . If floating or bouncing of the droplet did not the liquid. From conservation of energy, the penetration
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occur post collision, the impact will result in coalescence velocity is both calculated and experimentally verified to be
or splashing. Coalescence of the droplet in the liquid can be roughly half of the droplet impact velocity by Tran et al. .
[70]
partial or full. In partial coalescence, some of the droplets The penetration depth of a droplet impact into liquid, h,
merge into the liquid before “pinch off” occurs, where a is proportional to We [71] . In the case of bioprinting, a
secondary droplet is created and bounces off the surface just droplet contains particles, such as cells, which are jetted
like splashing. A critical Ohnesorge number determines onto a receiving substrate. When Park et al. injected cell-
[58]
where partial and full coalescence phase boundaries lie, laden droplets into a well containing cell culture media,
Oh* = 0.026 ± 0.001, while weakly associated with Bond they noticed that the cells sank to the bottom and attached
number, Bo = gDρ 2 [66] . to the liquid-filled substrate.
σ
Splashing produced by drop impact on liquid is In tissue engineering and bioprinting, hydrogel is
distinguished from splashing caused by droplet contact frequently employed as a carrier or substrate material for

Volume 9 Issue 5 (2023) 195 https://doi.org/10.18063/ijb.758