Dr Ahmed Ismail
PhD, MSc, BSc
My work focuses mainly on multiphase flow in micro-scale. This includes capillary jets, microdroplets and also electrohydrodynamics. The main objective of my research is to understand the physics in such small scale and then use this knowledge to develop different technologies . I use experimental, numerical and theoretical approaches to tackle challanges that arise from industy in areas such as 2D printing, addative manufacturing and micro-encapsulation.
- Electrostatic jet printing.
- Breakup of liquid jets.
- Dynamics of droplets.
Electrostatic jet printing is simply using an electric field to produce a fine jet. So, if we apply high enough voltage difference to a liquid coming out of a nozzle, the liquid will deform into a cone shape and a fine jet, much smaller than the nozzle in size will be produced out of apex. If we maintain a stable cone jet mode, we can print continuous pattern. We can also print small droplets simply by using a pulsating mode. A quick comparison between inkjet and Electrostatic jet printing shows how efficient the later one is in terms of deposition volume and the range of viscosity that can be printed.
In our lab we conduct series of studies to optimize both the print head design and the process conditions itself for acheiving high quality and resolution printing from different functional materials. For example, we managed to print up to 3 microns lines width from highly viscous nanoparticle ink (>5000 cp) for PV solar cells manufacturing and up to 600 nm pixel droplets for the manufacturing of high resolution OLED screens.
In this work a liquid droplet pends from an infinity plate and a second infinite plate is set in parallel at a distance. The two plates are connected to a voltage difference and the gap between these parallel plates is filled with a second fluid immiscible with the droplet. If the surrounding meduim is air, the electrical force overcomes capillary forces and a very fast microjet is emitted producing spray, but what if the outer medium is not negligible from the dynamic point of view as in the liquid meduim?
Our experimental and numerical observation show different modes. Depending on the operating parameters and fluid properties, a drop may result in the well-known tip streaming mode (with and without whipping instability), in droplet splitting (splitting mode), or in the development of a steep shoulder at the elongating front of the droplet that expands radially in a sort of “splashing” (splashing mode). We observed that these modes depend mainly on the viscosity of the outer medium.
We also characterized the conditions for each mode and the produced features size using the dimensionless analysis. This has a direct implementation in processes such as microencapsulation.
These results were published in Langmuir vol. 32, (27) 6815-6824.
The breakup of liquid jets in the capillary regime, which is caused by the growth of perturbations, has been a focus for many scientists over several decades owing to its importance in many chemical and manufacturing processes. In this work, we derived scaling laws of the intact length of an electrified jet in two different regimes, the viscous and non-viscous. The scaling laws are validated with experimental data obtained with a wide range of governing parameters.
These scaling laws are of a great interest, for example, to the printing sector. If the printing distance between the nozzle and the substrate is greater than the jet intact length, we could have scattered droplets around the line or non uniform lines. Therefore having scaling laws that can predict the jet intact length based on the properties of the used ink will allow us to set the printing distance to be less and hence ensure high quality printed lines.
This work was published in Physical Review Applied 10, 064010.
The controllable generation of small droplets and aerosols is of great importance in a large variety of technologies, ranging from drug delivery to microfluidics, crop spraying and inkjet printing. In particular, inkjet has been a key driver in the recent interest in droplet generation techniques as it is directly relevant to a variety of modern digital non-contact manufacturing processes, such as graphic printing, fabrication of transistors, biochip arraying, bioprinting and 3D printing.
The systematic parametric study carried out for this investigation included both experiments and numerical simulations to charactrize the velocity and the size of a droplet produced out of a controlled cavity collapse. The cavity collapse was achieved by creating pull-push-pull pressure pulse in a reservoir with a nozzle plate at its end. As a result, a fine jet is ejected out of the collapse and consequently a small droplet is separated from this jet. We conducted a theoretical analysis to predict the velocity and the size of the droplet based on the ejection conditions such as the pulse charactristics and the liquid properties. The theoretical models obtained from this study were validated by both experimental and numerical data showing an excellent agreement.
This work was published in Soft Matter, 14, 7671-7679.