1 Electric field -induced interfacial instability in a ferroelectric nematic liquid crystal Marcell Tibor Máthé12 Bendegúz Farkas12 László Péter1 Ágnes Buka1 Antal Jákli134

2025-04-30 0 0 958.78KB 17 页 10玖币

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Electric field-induced interfacial instability in a ferroelectric

nematic liquid crystal

Marcell Tibor Máthé1,2, Bendegúz Farkas1,2, László Péter1, Ágnes Buka1, Antal Jákli1,3,4,*,

Péter Salamon1,*

1Institute for Solid State Physics and Optics, Wigner Research Centre for Physics, P.O. Box

49, Budapest H-1525, Hungary

2Eötvös Loránd University, P.O. Box 32, H-1518 Budapest, Hungary

3Materials Sciences Graduate Program and Advanced Materials and Liquid Crystal Institute,

Kent State University, Kent, Ohio 44242, USA

4Department of Physics, Kent State University, Kent, Ohio 44242, USA

*: Author for correspondence: salamon.peter@wigner.hu

Abstract

Studies of sessile droplets and fluid bridges of a ferroelectric nematic liquid crystal in

externally applied electric fields are presented. It is found that above a threshold, the interface

of the fluid with air undergoes a fingering instability or ramification, resembling to Rayleigh-

type instability observed in charged droplets in electric fields or circular drop-type instabilities

observed in ferromagnetic liquids in magnetic field. The frequency dependence of the threshold

voltage was determined in various geometries. The nematic director and ferroelectric

polarization direction was found to point along the tip of the fingers that appear to repel each

other, indicating that the ferroelectric polarization is essentially parallel to the director. The

results are interpreted in connection to the Rayleigh and circular drop-type instabilities.

I. Introduction

Conventional nematic liquid crystals are usually formed by elongated organic molecules.

In a continuum description, the local symmetry axis of nematics is defined by the average

molecular orientation, which is called the director ()1. Nematic liquid crystals have already

achieved significant impact in our modern life as being the bases of the currently dominant

display technology. In flat display technology, where dielectric nematic liquid crystals are

utilized, the high frequency (AC) electric field-induced director reorientation results in the

electro-optical effect allowing to control the brightness of each pixel in a display2. An important

property of the nematic phase is the “head–tail” symmetry, i.e.,   . This head–tail

symmetry is broken in the ferroelectric nematic ( phase due to the spontaneous polarization









, where the director becomes a vector 





that is assumed to be parallel to 







3,4. The existence

of a ferroelectric liquid nematic phase was proposed already in 1916 5,6 by Max Born to explain

the isotropic-nematic phase transition. Although it turned out that ferroelectricity is not needed

for the existence of a nematic phase, scientists were looking for the  phase over a century,

but there were no unambiguous experimental indications of it until the syntheses of the highly

polar rod-shaped compounds referred to as DIO and RM734 by Nishikawa et al.7 and Mandle

et. al.8,9, respectively in 2017. The  phase of RM734 was first suggested to have splayed

polar order10–12, but more recently, it was shown that it has a uniform ferroelectric nematic

phase3. RM734 and DIO have large molecular dipole moments of about 10 D, and a

spontaneous polarization up to |







|~5 µC/cm2. The apparent dielectric permittivity and its

anisotropy were reported to be orders of magnitude higher than those of classical nematics,

reaching  ~ 104 or higher7,11,13–18 implying an extremely large sensitivity of the  materials

to electric fields. Such a large dielectric constant suggests a giant dielectrowetting at

unprecedentedly low voltages, since its threshold scales with the square root of the fluid’s

permittivity19–21. Furthermore, in analogy to the spectacular Rosensweig instability22 of

ferromagnetic fluids in magnetic field, one can expect to see electric field induced spike patterns

in ferroelectric fluids with free surface. A hint of such instability was demonstrated by Barboza

et al.23 who showed that sessile droplets of RM734 become unstable and disintegrate through

the emission of fluid jets when they are deposited on a lithium niobate (LN) ferroelectric crystal

substrate. This phenomenon was explained in analogy to the Rayleigh instability24 of charged

fluid droplets. Recent studies presented the behavior of sessile ferroelectric nematic droplets on

LN surfaces exposed to light25–28. We note that ferroelectric nematic droplets in various

environments29–32 exhibit an extraordinary tangential arrangement of the spontaneous

polarization.

In this paper, we reveal the nature of an AC electric field induced interfacial instability in

ferroelectric nematic sessile droplets and liquid bridges that resembles Rosensweig-type

instabilities observed in ferromagnetic liquids and Rayleigh-type instability observed in

charged droplets. In addition to quantitative measurements in various geometries, we will also

theoretically analyze the results.

II. Experimental results

We have studied the effect of AC electric fields in three different geometries: electric

voltage applied normal to the base plane of a sessile droplet (G1), and of a fluid bridge (G2),

and electric field applied along the base plane of a sessile droplet (G3). Unless it is indicated

differently, all measurements were carried out about 5 K below the    phase transition.

The phase transition temperature between the nematic and ferroelectric nematic phase was

.

The experimental geometry of G1 is shown in Figure 1a. Micrographs of a droplet

exposed to rms voltages   210 V, 220 V, and 300 V are seen in Figure 1b, 1c, and 1d,

respectively. The AC voltage at    frequency was applied between the base plate and

another ITO coated plate placed at    distance. At and below 210 V the droplet is

stable, while above it, the ejection of several thin jets can be observed. Some of them are

decorated with additional secondary jets like that observed by Barboza et al23. The structure is

stationary at a given voltage (see Supplementary Video 1).

Figure 1: A ferroelectric nematic sessile droplet in electric fields along the normal direction of the base plate. (a)

Illustration of the G1 geometry with approximate electric field (green lines) and coordinate system. (b-d)

Polarizing optical microscopy images of a ferroelectric nematic sessile droplet between crossed polarizers (yellow

arrows) and a full-wave plate (λ=546 nm - blue arrow) at 210 V, 220 V and 300 V, respectively. The cell gap was

L=150 µm.

The G2 geometry and a typical response to electric fields of an  bridge with   

thickness is shown in Figure 2. At high temperatures, in the isotropic and the nematic phase,

the circumferences of the contact lines on the bottom and top bounding plates start to grow

above a certain threshold voltage, while keeping their original round shape (Figure 2b). In

contrast to this behavior, in the ferroelectric nematic phase (Figure 2c) the contact line becomes

unstable and a fractal-like spreading of the fluid is observed (see Supplementary Video 2 for

another droplet). Inside the droplet at high voltage, electro-hydrodynamic convection takes

place that is stronger near the perimeter of the droplets. To precisely analyze the voltage

dependence of the electric field induced interfacial instability, we applied a geometrical

transformation, mapping a band around the contact line to a rectangle (Figure 2d), which we

will further discuss below. In Figure 2e, we show the side view of a liquid bridge exposed to

high voltage, clearly demonstrating that the instability forms in the vicinity of the glass plates.

We note that the visibility of the upper surface in the image in Figure 2e is blocked by the edge

of the upper substrate due to the inclined axis of observation.

Figure 2: A ferroelectric nematic fluid bridge in electric field along the normal direction of the base plate. (a)

Illustration of the G2 geometry. (b) Top view of a fluid bridge at zero voltage (558 µm diameter, 74 µm thickness)

and (c) at f =800 Hz, 52 V AC voltage. (d) Transformed view of the perimeter of the bridge exhibiting the instability.

Darker (lighter) pattern shows the instability at the top (bottom) glass. (e) Side view of the bridge clearly indicates

that the instability forms in the vicinity of the electrodes. The schematic illustration describes the experiment for

side-view imaging using a long-range microscope.

Using the transformed image of the perimeter of the droplet, we calculated a specific

type of roughness of the contact line denoted by  and defined in detail in the Methods

section.  gives a quantitative indicator for the emergence of the interfacial instability, while

being insensitive to the gradual growth of the circumference due to electrowetting.

Figure 3 shows the schematics of the in-plane electrode geometries (G3) together with

microphotographs of ferroelectric nematic droplets. Figure 3a illustrates the interdigitated

electrodes with the coordinate system, while Figure 3b and c show the side view (   plane)

of the electrodes with the fringing field, and the sessile droplet, respectively. The top-view ( 

 plane) of the reflection microscopy images of a droplet at zero voltage and with   ,

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摘要：

1Electricfield-inducedinterfacialinstabilityinaferroelectricnematicliquidcrystalMarcellTiborMáthé1,2,BendegúzFarkas1,2,LászlóPéter1,ÁgnesBuka1,AntalJákli1,3,4,*,PéterSalamon1,*1InstituteforSolidStatePhysicsandOptics,WignerResearchCentreforPhysics,P.O.Box49,BudapestH-1525,Hungary2EötvösLorándUniversi...

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1 Electric field -induced interfacial instability in a ferroelectric nematic liquid crystal Marcell Tibor Máthé12 Bendegúz Farkas12 László Péter1 Ágnes Buka1 Antal Jákli134

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