Engineering and Architecture
Alexandre Fabregat Tomàs
Nanoscience, Materials and Chemical Engineering
Rheological study of droplet formation in biofluids
Liquid droplet formation is relevant to several applications where the deposition of a controlled volume of fluid on a specific location is required. Different techniques can be employed to form a droplet and piezo-type Drop-on-Demand is one of the most recent. This method consists in creating a pressure wave within a micro-capillary channel full of liquid with an orifice at one end with a typical diameter of 20 to 50 μm. Piezoelectric wall actuators generate a pressure wave which acts against the fluid viscosity and surface tension. A ligament of liquid is ejected from the orifice and subsequently breaks into droplets.
It is well known that droplet formation is influenced by both the physical properties of the ejected fluid (surface tension, viscosity and density) and the flow conditions characterized by a length l and a velocity u scales. The amplitude of the pulse applied on the fluid also influences the jetting by modifying the fluid velocity of the droplet outside the microdispencer. From dimensional analysis, the Reynolds number Re, which represent the ratio of the inertia to viscous force, the Weber number We, which represent the ratio of the inertia to surface tension force, and the Ohnesorge number Oh = We/Re, which represents the ratio of viscous to surface tension forces and which is independent of the droplet velocity, have been found to control the droplet formation process. Theoretical studies indicate that stable droplet formation can occur for La > 2 where La = 1/Oh is the Laplace number defined as the inverse of the Ohnesorge number. Computational and experimental study about the DOD drop formation also showed that a printable fluid should obey 1 < La < 10. The lowest value of Z is governed by the dissipation of the pressure wave by the viscosity of the fluid whereas the higher limit is determined by the fact that the fluid forms satellites droplet instead of a unique droplet. These studies of drop-on-demand jetting have only considered Newtonian fluids, however, it has been established that viscoelasticity can strongly influence droplet velocity and droplet volume. Fluid linear viscosity can be represented in term of a complex viscosity η* = (G’ + iG’’)/ω where G’ is the elastic modulus and G’’ is the loss modulus. Previous work has shown that drop-on-demand dispensed fluids can be characterised using G’ and G’’ data, however, the short relaxation times involved with drop-on-demand fluids require special experimental techniques to be adopted.
In this thesis we aim at studying the way linear and nonlinear viscoelastic properties of the fluids affect the filament breakdown and droplet formation mechanism. Toward this end, we need to perform two measurements:
First, we need to identify the rheological properties of the fluid examined than we need to observe how this fluid behave when is doped with particles/proteins and is dispensed true a drop-on-demand microdispenser. For this purpose, we need to test the micro-dispensing behaviour of the fluid previously characterised with the rheological test.
Ethics: This project does not involve ethical aspects.
Workplace location: Campus Sescelades, Tarragona
37.5 hours a week
15 March 2021
|This project has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 945413|