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Demixing in simple fluids induced by electric field gradients


Phase separation in liquid mixtures is mainly controlled by temperature and pressure, but can also be influenced by gravitational, magnetic or electric fields. However, the weak coupling between such fields and concentration fluctuations limits this effect to extreme conditions1,2,3. For example, mixing induced by uniform electric fields is detectable only at temperatures that are within a few hundredths of degree or less of the phase transition temperature of the system being studied4,5,6,7. Here we predict and demonstrate that electric fields can control the phase separation behaviour of mixtures of simple liquids under more practical conditions, provided that the fields are non-uniform. By applying a voltage of 100 V across unevenly spaced electrodes about 50 µm apart, we can reversibly induce the demixing of paraffin and silicone oil at 1 K above the phase transition temperature of the mixture; when the field gradients are turned off, the mixture becomes homogeneous again. This direct control over phase separation behaviour depends on field intensity, with the electrode geometry determining the length-scale of the effect. We expect that this phenomenon will find a number of nanotechnological applications, particularly as it benefits from field gradients near small conducting objects.

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Figure 1: Phase diagram of a symmetric A/B liquid mixture.
Figure 2: Wedge-shaped model system.
Figure 3: Phase separation with razor-blade electrodes.
Figure 4: Temperature and voltage dependence of phase separation.

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We thank D. Andelman and P. G. de Gennes for discussions and suggestions, and R. Bonnecaze and A.-V. Ruzette for comments on the manuscript. We gratefully acknowledge support from ATOFINA.

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Correspondence to Ludwik Leibler.

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The authors declare that they have no competing financial interests.

Supplementary information

Supplementary Animation 1

Temperature induced phase separation: The Paraffin-silicone oil mixture (42.7% w/w of squalane in polymethylphenylsiloxane) is cooled down (-0.1°C/mn) from above to below the phase separation temperature Tt= 80.4°C. No voltage is applied, droplets appear throughout the sample. (AVI 3294 kb)

Supplementary Animation 2

Electric field gradient induced phase separation: Same sample at T=Tt + 0.1 °C (homogeneous state). A voltage (500 V a.c.) is applied across ITO transparent electrodes separated by a 50µm gap. A silicone rich channel appears along the electrode edges. (AVI 1032 kb)

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Tsori, Y., Tournilhac, F. & Leibler, L. Demixing in simple fluids induced by electric field gradients. Nature 430, 544–547 (2004).

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