The retraction of jetted slender viscoelastic liquid filaments

Long and slender liquid filaments are produced during inkjet printing, which can subsequently either retract to form a single droplet, or break up to form a primary droplet and one or more satellite droplets. These satellite droplets are undesirable since they degrade the quality and reproducibility of the print, and lead to contamination within the enclosure of the print device. Existing strategies for the suppression of satellite droplet formation include, among others, adding viscoelasticity to the ink. In the present work, we aim to improve the understanding of the role of viscoelasticity in suppressing satellite droplets in inkjet printing. We demonstrate that very dilute viscoelastic aqueous solutions ( $\text {concentrations} \sim 0.003\,\%$ wt. polyethylene oxide, corresponding to nozzle Deborah number $De_{n}\sim 3$ ) can suppress satellite droplet formation. Furthermore, we show that, for a given driving condition, upper and lower bounds of polymer concentration exist, within which satellite droplets are suppressed. Satellite droplets are formed at concentrations below the lower bound, while jetting ceases for concentrations above the upper bound (for fixed driving conditions). Moreover, we observe that, with concentrations in between the two bounds, the filaments retract at velocities larger than the corresponding Taylor–Culick velocity for the Newtonian case. We show that this enhanced retraction velocity can be attributed to the elastic tension due to polymer stretching, which builds up during the initial jetting phase. These results shed some light on the complex interplay between inertia, capillarity and viscoelasticity for retracting liquid filaments, which is important for the stability and quality of inkjet printing of polymer solutions.

Drop-on-demand cell bioprinting via Laser Induced Side Transfer (LIST)

We introduced and validated a drop-on-demand method to print cells. The method uses low energy nanosecond laser (wavelength: 532 nm) pulses to generate a transient microbubble at the distal end of a glass microcapillary supplied with bio-ink. Microbubble expansion results in the ejection of a cell-containing micro-jet perpendicular to the irradiation axis, a method we coined Laser Induced Side Transfer (LIST). We show that the size of the deposited bio-ink droplets can be adjusted between 165 and 325 µm by varying the laser energy. We studied the corresponding jet ejection dynamics and determined optimal conditions for satellite droplet-free bioprinting. We demonstrated droplet bio-printing up to a 30 Hz repetition rate, corresponding to the maximum repetition rate of the used laser. Jet ejection dynamics indicate that LIST can potentially reach 2.5 kHz. Finally, we show that LIST-printed human umbilical vein endothelial cells (HUVECs) present negligible loss of viability and maintain their abilities to migrate, proliferate and form intercellular junctions. Sample preparation is uncomplicated in LIST, while with further development bio-ink multiplexing can be attained. LIST could be widely adapted for applications requiring multiscale bioprinting capabilities, such as the development of 3D drug screening models and artificial tissues.

“Dancing Droplets”: Partial Coalescence on Superhydrophobic Surfaces

Droplet coalescence has received significant attention due to its significant role in fluid mixing, microfluidics, coalescence-induced droplet jumping, and heat and mass transfer applications. Coalescence of droplets has been extensively investigated from the perspectives of hydrodynamics and energy transfer. However, the study of coalescence characteristics of size-mismatched droplets on superhydrophobic surfaces remains a challenge due to visualization difficulty, limited droplet size control, and poor droplet manipulation. Here, in order to study coalescence dynamics of droplets with arbitrary initial sizes, a droplet dispensing and visualization system was developed. To control the size of droplets, monodispersed droplets with radii of ≈20 μm were dispensed using a frequency-controlled piezoelectric pulse injector onto a superhydrophobic surface, enabling the target droplets to accumulate in volume and grow in radii. The coalescence process of droplets having radii of ≈270 and ≈780 μm was imaged at a magnification of ≈25X and capture rate of 13000 fps. Surprisingly instead of completely merging together, the size-mismatched droplets underwent partial coalescence with the development of an additional satellite droplet. Specifically, the smaller droplet gave 'birth' to a secondary satellite droplet upon coalescence with the larger primary droplet due to liquid-bridge pinch-off dynamics, after which the satellite droplet bounced off upon collision with the primary droplet due to the presence of an air cushion that blocked contact between the two droplets. Meanwhile, the primary droplet continued to oscillate while the bouncing satellite droplet returned to the surface and eventually bounced off (moving direction is identified with arrows). Our work not only presents a powerful platform capable of both controlling and visualizing droplet coalescence hydrodynamics, but also provides insights into the flow hydrodynamics of droplets undergoing partial coalescence.

Turning on/off satellite droplet ejection for flexible sample delivery on digital microfluidics

Convenient electric control and electrode design allow flexible sample delivery on-chip in a wide range on microfluidics.

Simulation & modelling of dilute solutions in drop-on-demand inkjet printing: a review

This review is about the most important matters for advancing inkjet printing with a focus on piezoelectric droplet on demand (DOD) inkjet thin-film devices. The Nano material compounds can be incorporated into a polymeric matrix and deposited by piezoelectric inkjet printing. Current problems in advanced printers are discussed as embodied in liquid filament breakup along with satellite droplet formation and reduction in droplet sizes. Various model that predicts the printability of dilute, mono disperse polymer solutions in drop-on-demand “DOD” inkjet printing have been discussed. For satellite droplets, it is exhibited which liquid filament break-up treatment can be predicted via using a combination of two pi-numbers, including the Weber number. The layer was printed over other printed layers including electrodes composed of the conductive polymers and also several polymers. It has been discussed, some polymer materials are suitable for deposition and curing at low to moderate temperatures and atmospheric pressure, allowing for the use of polymers or paper as supportive substrates for the devices, and greatly facilitating the fabrication process. Furthermore, through this review, it has been discussed scaling analyses for designing and operating of inkjet heads. Because of droplet sizes from inkjet nozzles are typically on the order of nozzle dimensions, a numerical simulation is shown for explaining how to reduce droplet sizes through employing a novel input waveform impressed on the inkjet-head liquid inflow without changing the nozzle geometries. Regardless of their any less performance, inkjet printer head as a technique for the mentioned devices presents many advantages, the most notable of which are quickly fabricating and patterning, substrate flexibilities, avoidance of material wastage via applying “DoD” technologies.

Waveform adjustment for obtaining higher quality droplets based on a multi-satellite droplet monitoring system

In the field of printed electronics, there are many kinds of ink with a variety of performance and parameters. In order to generate high-quality droplets, it is necessary to eject droplet for testing in order to discover the appropriate excitation voltage and waveform shape. Consequently, a real-time and efficient droplet monitoring system is needed to monitor droplet formation quality. A multi-satellite droplet monitoring system is proposed in this paper. It can not only monitor the droplet with multiple satellites but also can generate the instantaneous droplet position curve, length curve and velocity curve. The waveform voltage and dwell time can be adjusted in real-time to obtain droplets with higher quality and efficiency.

On the breakup of spiralling liquid jets

The generation of drops from a jet spiralling out of a spinning device, under the action of centrifugal force, is considered for the case of small perturbations introduced at the inlet. Close to the inlet, where the disturbances can be regarded as small, their propagation is found to be qualitatively similar to that of a wave propagating down a straight jet stretched by an external body force (e.g. gravity). The dispersion equation has the same parametric dependence on the base flow, but the base flow is, of course, different. Further down the jet, where the amplitude of the disturbances becomes finite and eventually resulting in drop formation, the flow appears to be quite complex. As shown, for the regular/periodic process of drop generation, the wavelength corresponding to the frequency at the inlet, increasing as the wave propagates down the stretching jet, determines, in general, not the volume of the resulting drop but the sum of volumes of the main drop and the satellite droplet that follows the main one. The proportion of the total volume forming the main drop depends on how far down the jet the drops are produced, i.e. on the magnitude of the inlet disturbance. The volume of the main drop is found to be a linear function of the radius of the unperturbed jet evaluated at the point where the drop breaks away from the jet. This radius, and the corresponding velocity of the base flow, have to be found simultaneously with the jet’s trajectory by using a jet-specific non-orthogonal coordinate system described in detail in Shikhmurzaev & Sisoev (J. Fluid Mech., vol. 819, 2017, pp. 352–400). Some characteristic features of the nonlinear dynamics of the drop formation are discussed.