| Our
main research focuses on electrified liquid interfaces (electrosprays)
and ultrafine particles. We are interested in techniques for producing
ultrafine particles from electrosprays and in instrumentation to analyze
nanometer particles, such as differential mobility analyzers (DMA's),
condensation nucleus counters (CNC's) and inertial impactors. An
important component of our current electrospray work has to do with the
formation of Taylor cones in a vacuum for production of ion beams for
electrical propulsion in space.
Electrosprays
The photographs below depict the interface between a gas and an
electrically conducting liquid charged to several kV. The familiar
rounded meniscus that would hang on the left end of the tube has been
transformed into a cone through the balance between surface tension and
electrical forces. Most interesting is the fact that the cone apex emits
a micro-jet (right) whose diameter can be controlled from atomic
dimensions (in the case of liquid metals) up to hundreds of micrometers
(for nearly insulating liquids). This jet then breaks up into a spray or
electrospray of nearly identical charged droplets (left).
We have learned much on how to control the
dimensions of the jet. At present, no other approach is capable of
dividing liquids as finely as this. We are pursuing a variety of related
exciting engineering applications and basic problems. For example, when
we apex region of the cone reaches scales of 10 nm, individual ions
begin to be ejected from the liquid into the gas by field evaporation.
Of particular current interest is the fact that the electrosprays of
certain materials (including salts that melt below room temperature,
also called ionic liquids) held in a vacuum yield only ions without any
drops. This purely ionic regime is similar to that exhibited by liquid
metals, but is open to a physicochemically much richer group of
materials. Our main thrust in this area is driven by space propulsion
applications, in collaboration with the companies Busek, Inc. (Natik,
MA), Connecticut Analytical (New Haven, CT) and with Prof. Martinez
Sanchez (MIT). Currently we have a position for a graduate and
several undergraduate students for studies on electrical propulsion
based on and are open for a NASA-funded project.
Ultrafine
Particles
Our laboratory has been active for over a decade in the study of gas
suspensions of particles in the nanometer size range. We have developed
the only available condensation nucleus counters (CNC) capable of
detecting individual ions and nanoparticles of subnanometer dimensions
(J. Aerosol Sci., 31, 757-772, 2000; Aerosol Sci. & Tech., 38, 1-11,
2004). Together with the novel Eichler and Herrmann DMAs (see end of
page), this CNC has allowed some rather exciting studies on ions and
clusters produced by electrospray and has led to much progress on the
problem of the origin of electrospray ions.
We have developed
several high resolution techniques to analyze nanoparticles down to the
region of clusters and large molecules. One approach uses inertial
impactors in high speed flows, which is limited to sizes above 5 nm
except for dense particles where 2 nm diameters are analyzable. Some of
these impactors use the phenomenon of aerodynamic focusing to achieve
unusually high resolution. See references below.
Differential
Mobility Analyzers (DMAs)
State of the art DMAs offer high resolving power down to molecular
dimensions. Several such instruments have been developed at Yale by Joan
Rosell, Luis de Juan, Thilo Eichler, Wolfgang Herrmann, and by
colleagues at CIEMAT (Madrid) and RAMEM (Madrid) See references below. 
These
DMA's operate at high speeds and remain laminar at Reynolds numbers Re
in excess of 30,000. The figure above shows spectra obtained with the
Herrmann DMA with ions roughly 2 nm in diameter. The various peaks are
obtained at different Reynolds numbers, showing initially a resolving
power increasing with Re (higher mean peak voltage) up to the onset of
turbulence, at Re ~ 35,000,
seen in the figure at about 5000 volt. The unique features of these DMAs have
led to numerous requests from other groups wishing to attain similar
resolving power in the nanometer range. The following remarks are
intended to deal with such requests.
The main feature of the Eichler DMA which allows
laminar operation at such high Reynolds numbers is a rather large
laminarization inlet trumpet which is protected by Yale patents licensed
to TSI Inc. ("Method and apparatus for separating ions in a gas for
mass spectrometry." U.S. patent 5,869,831 and 5,936,242; J. Fernández
de la Mora, L. de Juan, T. Eichler, and J. Rosell). Any interested party
is, therefore, encouraged to contact TSI directly regarding availability
date, price, size, range, and resolving power. TSI has had no access to
our group's experience on building such instruments, so that none of the
performance data from our published papers will necessarily apply to
their instrument when it becomes available. For sales outside US
territory contact RAMEM (Madrid).
- Selected
Publications:
- "Cluster
ion formation in electrosprays of acetonitrile seeded with ionic
liquids," Bon Ki Ku and J. Fernandez de la Mora, J. Phys.
Chem, (in press 2004).
-
- "Spatial
structure and energy distribution of electrosprays of ionic
liquids in vacuo," I. Romero-Sanz and J. Fernandez de la
Mora, J. Applied Physics, 95(4), 2123-2129 (2004).
-
- "Ammonium
electrolytes quench ion evaporation in colloidal propulsion,
Rodrigo Bocanegra, Juan Fernandez de la Mora and Manuel Gamero, Journal
of Propulsion and Power, 20(4), 728-735 (2004).
-
- "A
drop-free ion source from Taylor cones of ionic liquids," I.
Romero, R. Bocanegra, J. Fernandez de la Mora, and M.
Gamero-Castaño, J. Appl. Phys., 94, 3599, 3605
(2003).
References on the fundamentals of
electrosprays
References on electrospray
ionization
References on DMA's
References on aerodynamic focusing
References on inertial impactors
References on general
nanoparticle work
References on fluid dynamics |
