Prof. YoavTsori
Department of Chemical Enginnering and the Ilse Katz Institute for Nanoscale Science and
Ben-Gurion University of the Negev
Tel +972-8 6477794
Fax +972-8 6462916
Laboratory for phase transitions in electric field gradients
Ben-Gurion University of the Negev
Theory or experiments of phase transitions in electric fields; chemical and biochemical reactions in
field gradients.
Language – English preferred. Background – Ph.D. in physicsor chemical, mechanical, material or
electrical engineering. For theory – knowledge in phase transitions, electrostatics, various analytical
methods. For experimental work – experience in electronic circuits, oscilloscopes and other electronic
components, background in thermometric measurements, phase transitions, and optical methods in
particular optical microscopy (e.g. confocal microscope).
Any date
French Embassy in Israel
Office for Science & Technology
Rothschild Boulevard 7
Tel Aviv, 66881 Israel
At least 12months
Tel: +972 (0)3 796 80 42
Fax: +972 (0)3 796 80 45
Web :
Email :
(Pleaseencloseatwopagesdescriptionoftheproject – Arial10,Linespacing1.5point)
Phase transitions and chemicalreactionsin simple liquids in electricfield gradients
When a mixture of two simple liquids, or a pure liquid in coexistence withitsvapor, are under the influence of
spatiallyuniformelectricfield, the criticaltemperaturemay change by a smallamount, typically in the mKregime.
A different scenario occurswhen the externalfields has gradients. In this case the change to the coexistence
temperatureis 2-100 largerthan the change in uniformfields. The phase separationisreversible: whenfield
gradients are turned off, the mixture becomeshomogeneousagain. We are investigatingthis new
“electroprewetting” by pursuinganalyticalcalculations, extensive computer simulations and experiments.
Fig. 1. Field-induced phase transition in mixtures of simple liquids. (a) Field off. Two flat electrodes are coveredwithhomogeneous
mixture of siliconoil and paraffin. (b) Field on. Silicon-richdomainsform close to the electrodes' edges, paraffin-rich phase iselsewhere.
Bar is 50 m [Nature430, 544 (2004)].
We base ourtheoretical investigations on a coarse-grainedmean-field free energydensity of the form
wherefmis the mixture free energy, typicallyhaving a form of a double-well for temperaturebelow T c, and is
the mixture composition (0<<1), () is the dependence of the local dielectric constant on , and fentropy and
fsolvation are the entropydensity and solvationenergy of the ions in the mixture. This formulation allowed us to
recentlystudy (i) liquid-vapour coexistence in pure and dielectricmaterials (Fig. 2), (ii) interactions
betweencolloidsimmersed in a binary mixture containingsalt (Fig. 3), and (iii) the dynamics of phase
separation in a cylindricalcapacitor (Fig. 4).
French Embassy in Israel
Office for Science & Technology
Rothschild Boulevard 7
Tel Aviv, 66881 Israel
Tel: +972 (0)3 796 80 42
Fax: +972 (0)3 796 80 45
Web :
Email :
Fig. 2. Liquid-vapour coexistence in a
quadrupolar system. Wires (white circles)
are chargedwithalternatingsigns. Red:
liquid, blue: vapour.
Outercircleisgrounded. J. Phys. Chem. B
115, 75 (2011).
Fig. 3. The potential vs separation D
betweentwochargedcolloidssuspended in a
binary mixture containingsaltatdensity n0.
eis the surface charge density of the
colloids. EPL 95, 36002 (2011).
Fig. 4. Dynamics of phase separation in
abinary mixture of twoliquids (blue and red)
inside a cylindricalcapacitor. Innercylinder
(white) ischarged and the outer one
isgrounded. Weused a model B dynamicswith
the free energyabove.
Preliminaryexperiments have revealedseveralinterfacialinstabilities, occurring as a competitionbetween
surface tension and electrostatic forces. In addition, as the size of the electrodesisreduced, surface tension
becomes more dominant; at the scale of 50 m we are able to achieve a large array of small drops of one
liquidembedded in a matrix of the second liquid. This array of droplets of one solventembedded in a second
solventis the basis of the idea of how to control the rate and extent of chemicalreactions: if
tworeagentsundergo a reaction, when the solvents are mixed the reactionwilltake place everywhere.
Whenthey are demixed, however, the reactionwillbeaccelerated and willtake place only in a
smalldropletwhich serves as a micro-reactor.
The phase transition may have various applications in the nanotechnological world, sinceitbenefitsfromfield
gradients nearsmallconductingobjects. Some of the promising directions studied by us are demixing in
microfluidicschannels, MEMS, and electrolubrication.
Fig. 5. Illustration of phase separation in
microfluidicschannels. A homogeneous mixtures
flows to the left. Suitablyarrangedelectrodes
(yellow) lead to demixing of the twofluids (cyan
and pink) which continue to flow in
differentchannels. We are studying the basic
physicalprocesstheoretically and experimentally.
French Embassy in Israel
Office for Science & Technology
Rothschild Boulevard 7
Tel Aviv, 66881 Israel
Tel: +972 (0)3 796 80 42
Fax: +972 (0)3 796 80 45
Web :
Email :