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This project is a patient study
of the mechanisms responsible for various electrohydrodynamic (EHD)
phenomena, with the ultimate intend of applying these flows to the generation
of particles with diameters well under 1 micron. Through a rigorous
experimental investigation, we will contribute fundamental new knowledge
in the field, and to the development of novel measurement techniques
of these invisible particles. This project offers the opportunity to
develop strong scientific enquiry skills and other technical skills
that are highly valued across academia and industry.
Background
and State of the Art:
Spray formation is the starting step in
the manufacture of solid particles by pyrolisis and by spray drying.[1,2]
Very small, submicron droplets (diameter <100 nm typ.) can be used
to generate dry particles in the 1-20 nm of interest in nanotechnology
research and applications. Electrostatic spraying and electrospinning
result from the interaction between strong electric fields and liquid
surfaces. Typically, a conductive liquid is pulled by the electric field
into a “cone-jet”, where a liquid jet is ejected from the
tip of a conical meniscus called “Taylor cone” [3,4]. Jets
that break up yield electrically charged, non-coalescing, “electrospray”
droplets, whereas jets that don’t break up form charged fibers
(typ. under high viscosity conditions). By proper adjustment of the
properties of the liquid, the diameter of these droplets and fibers
can be “tuned” anywhere from 100’s of microns down
to atomic dimensions. These flows have led to some very important applications,
which exploit the fact that electrospray droplets have a narrow size
distribution, are highly charged and non-coalescing. For example, “liquid
metal ion sources” produce atomic ions and cluster ions (“droplets”)
from very highly conductive liquids like Gallium, and are the very bright
sources used for focused ion beam (FIB) milling in semiconductor manufacturing
[5,6] Electrosprays have enabled the use of mass spectrometric methods
for rapid and accurate determination of molecular weights of protein
ions in so called “electrospray ionization mass spectrometry”
(ESI-MS). [7,8] This contribution to the current proteomics revolution
has motivated the 2002 Nobel Prize in Chemistry awarded to Dr John B
Fenn (http://www.nobel.se/chemistry/laureates/2002/fenn-lecture.html).
EHD flows are currently also being proposed as thrusters for attitude
control of microsatellites, for fiber electrospinning in tissue engineering,
for respiratory drug delivery, and as particle size standards in the
sub-micrometric range, among other uses. Much work has been carried
out over the years to try to understand the transport mechanisms (mass,
momentum, energy, and charge) underlying electrospraying, [9] but most
of it has centered on the “steady cone-jet” mode. Other
modes have not received nearly the same level of attention, and are
therefore far less well understood, in particular so called “nanospray”
[10], and “corona-assisted electrospray” [11].
Problem Statement
and Methodology:
Our ultimate goal is to modify
known electrohydrodynamic flows to make them more adequate for the manufacture
of nanoparticles, so as to retain their aforementioned advantages while
overcoming their traditional shortcomings. These shortcomings are: A)
having to tune the liquid properties in order to achieve a certain drop
size (e.g. raising the electric conductivity -needed for small droplet
sizes- is not always feasible); B) the window of stable flow rates for
a given liquid composition is very narrow; C) the flow rate needs to
be very small in order to obtain the very small particles of interest
here; and D) the droplet size distributions broaden as the mean droplet
size is reduced. While these circumstances are compatible with some
applications, [12] they are limiting in many areas, such as nanotechnology,
where exquisite control over particle size is required, but high yields
are also needed.
We have identified a number of
experimental studies aimed at elucidating why these limitations exist,
and how they could be overcome. Our methods will include approaches
to extract information about transport mechanisms from the behavior
of the stability islands of system responses (e.g. droplet size standard
deviation). Rapid measurement techniques for determining particle size
and electrostatic charge of the submicron droplets will be applied.
This is a key aspect of the project, which we plan to tackle via adoption,
probably also improvement, of aerosol measurement techniques (for electrical
mobility, aerodynamic diameter, and charge-to-mass ratio). Instrumentation
development for nanometer size particles is a vibrant field of research
[13]. This is an experimental project, but it is expected that the work
will link to other ETSEQ researchers interested on computational aspects
of these phenomena.
Our FeT team initiates this program
for the first time this year with the incorporation of Dr Joan Rosell
from ICREA (http://www.etseq.urv.es/fet/321.html).
For this reason we will be setting up a new lab, and this is an opportunity
to provide day-to-day leadership in the making of our new nanofluidics
lab.
The
ideal candidate:
It is expected that the candidate will have a Bachelor’s
degree or equivalent in Chemical Engineering, Mechanical Engineering,
Physics or Chemistry. Other requirements are: solid understanding of
physics fundamentals, mechanical aptitude (ability for handling and
assembling small mechanical components), excellent organizational and
communication skills, proficiency in English, and enthusiasm for research
and learning.
Finishing this
project:
The person working on this project
will develop very strong scientific enquiry skills and other technical
skills that are highly valued across academia and industry. The graduate
will enjoy the multidisciplinary environment of one of the best Ph.D.
programmes in Spain.
References and
notes:
[1] TT Kodas and MJ Hampden-Smith,
Aerosol Processing of Materials, Wiley-VCH (1999).
[2] SE Pratsinis and TT Kodas, in Aerosol Measurement, Willeke
K and Baron PA Eds., Chap 33, Van Nostrand Reinhold (1993).
[3] G Taylor, Disintegration of water drops in an electric field,
Proc. Roy. Soc. A, 280:383-397, (1964).
[4] M. Cloupeau and B. Prunet-Foch, Electrohydrodynamic Spraying
Functioning Modes: A Critical Review, J. Aerosol Sci., 25(6):
1021-1036 (1994).
[5] J Orloff, Focused Ion Beams, Sci. Am., Oct 1991,: 96-101
(1991).
[6] J Orloff, L Swanson, M Utlaut, High Resolution Focused Ion
Beams. FIB and Applications, Kluwer Academic/Plenum Publishers (2002).
[7] JB. Fenn et al., Electrospray ionization for mass spectrometry
of large biomolecules, Science, 246: 64-71 (1989).
[8] A Pandey and M Mann, Proteomics to Study Genes and Genomes,
Nature, 405: 837-405 (2000).
[9] J Rosell-Llompart and J Fernández de la Mora, Generation
of monodisperse droplets 0.3 to 4 um in diameter drom electrified con-jets
of highly conducting and viscous liquids, J. Aerosol Sci. 25:
1093-1119 (1994).
[10] M Wilm, M Mann, Analytical properties of the nanoelectrospray
ion source, Anal Chem, 68(1): 1-8 (1996).
[11] K Tang and A Gomez, Generation of monodisperse water droplets
from electrosprays in a corona-assisted cone-jet mode, J. Coll. Interface
Sci, 175:326-332, (1995)
[12] For instance, in ESI-MS the goal typically is to use as little
sample size as possible while retaining high signal-to-noise ratios
in the mass spectrometer
[13] J. Fernández de la Mora et al., Differential Mobility
Analysis of Molecular Ions and Nanometer Particles, Trends in Analytical
Chemistry, 17(6): 328-339 (1998)
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