SUPERSCREENING AND POLARIZATION CONTROL IN CONFINED FERROELECTRIC NEMATIC LIQUIDS Federico Caimi1 Giovanni Nava1 Susanna Fuschetto1 Liana Lucchetti2 Petra Paiè3 Roberto Osellame4 Xi Chen5

2025-05-02 0 0 5.89MB 12 页 10玖币

侵权投诉

SUPERSCREENING AND POLARIZATION CONTROL IN CONFINED

FERROELECTRIC NEMATIC LIQUIDS

Federico Caimi1, Giovanni Nava1, Susanna Fuschetto1, Liana Lucchetti2, Petra Paiè3, Roberto Osellame4, Xi Chen5,

Noel A. Clark5, Matthew Glaser∗5*, and Tommaso Bellini†1*

1Dept. of Medical Biotechnology and Translational Medicine, University of Milano, Italy

2SIMAU Dept., Università Politecnica delle Marche, Ancona, Italy

3Dipartimento di Fisica, Politecnico di Milano, Milano, Italy

4Istituto di Fotonica e Nanotecnologie, Consiglio Nazionale delle Ricerche (IFN-CNR), Milano, Italy

5Dept. of Physics, Soft Materials Research Center, University of Colorado, Boulder, CO, USA

ABSTRACT

The combination of large spontaneous polarization and ﬂuidity makes the newly discovered ferroelec-

tric nematic liquid crystalline phase (N

) responsive to electric ﬁelds in ways that have no counterpart

in other materials. We probe this sensitive ﬁeld response by conﬁning a N

ﬂuid in microchannels

that connect electrodes through straight and curved paths. We ﬁnd that by applying electric ﬁelds

as low as E

≈

0.5 V/mm, the N

phase orders with its polarization smoothly following the winding

paths of the channels even when oriented antiparallel to the line connecting positive to negative

electrodes, implying analogous behavior of the electric ﬁeld. Upon inversion of E, the polar order

undergoes a complex multistage switching process dominated by electrostatic interactions. Multistage

polarization switching dynamics is also found in numerical simulations of a quasi-2D continuum

model of N

liquid crystals in microchannels, which also clarify the conditions under which the

electric ﬁeld is guided by the microchannels. Experiments and theory indicate that all observations

are direct consequences of the prompt effective screening of electric ﬁeld components normal to the

channel walls. This electric “superscreening” effect emerges as a distinctive property of the N

phase,

capable of inducing conditions in which both the polarization and the electric ﬁeld are guided by

microchannels.

The discovery of molecules capable of ordering into a ferroelectric nematic (N

) liquid crystal (LC), a state predicted

a century ago [

] but only recently observed [

], has attracted immediate interest because of the novel

properties of this new ﬂuid state, involving its electric behavior, its elasticity, and its non-linear susceptibilities

[

]. Conventional nematic LCs are non-polar anisotropic ﬂuids, in which molecules partially align

along a common axis, the so-called nematic director

. In the N

phase, a bulk electric polarization density

develops

parallel to

through a weak ﬁrst order phase transition. Molecules found to give rise to N

ordering have large

permanent electric dipoles (

>10

D) and a high degree of polar orientational order, resulting in a large ferroelectric

polarization, up to P0=|P| ≈ 6µC/cm2[6, 14, 15].

The presence of a bulk polarization ﬁeld that is readily reoriented makes the electric ﬁeld response of (N

) markedly

different from the dielectric response of conventional nematics, in ways that have yet to be fully explored. Because of

the dielectric anisotropy of conventional nematics, the electric ﬁeld (

) has a tensorial coupling to

, producing torques

on the director proportional to

, and of a magnitude comparable to the orientational elasticity. In the N

phase the

presence of a bulk polarization gives rise to a dipolar coupling to

and self-interaction of space charges arising from

divergence of the polarization, producing electric torques orders of magnitude larger. Indeed, because of the ﬂuidity of

∗matthew.glaser@colorado.edu

†tommaso.bellini@unimi.it

arXiv:2210.00886v1 [cond-mat.soft] 3 Oct 2022

Superscreening and polarization control in conﬁned ferroelectric nematic liquids

the N

phase, the orientation of

is a Goldstone variable with energetics dominated by electrostatics, with nematic

elasticity and surface anchoring playing a minor role. The unconstrained rotation of

is affected only by the weak

elasticity arising from its non-uniform orientation and by the self-interaction caused by electrostatic forces arising from

polarization space charge at surfaces (

σP=P·u

being the unit vector normal to a limiting surface of N

) and in the

bulk (ρP=−∇ · P).

All these factors combine to create a fundamentally new kind of ﬂuid, markedly different from conventional nematic

LCs and from solid ferroelectric materials, whose polarization, intrinsically constrained by the symmetry of the crystal

unit cell, is typically limited to a set of easy axis orientations. In this work we explore the interplay of external ﬁelds,

geometry, and electrostatic self-interaction N

ﬂuids conﬁned within straight and bent microchannels. Indeed, because

of its spontaneous bulk polarization, a ferroelectric nematic ﬂuid conﬁned to a channel should, in the absence of

external electric ﬁelds and other couplings, align with

along the channel to minimize the overall electrostatic energy,

a behavior with intriguing implications in the case of bent channels.

Description of the experiment

We have produced, via femtosecond laser micromachining assisted by chemical etching [

], microchannels

buried in monolithic fused silica that connect gold wire electrodes. The channels, shown in Fig. 1a-b, are all of length

`≈1 mm

and have a rounded square cross-section of width

w≈35 µm

. Four different shapes are produced: straight

along the x-axis direction connecting the electrodes (I-shaped channel), with the central part bent with respect to the

x-axis of

90◦

(L-shaped channel),

135◦

(Z-shaped channel) and

180◦

(S-shaped channel) (see Fig. 1b). The channels

were ﬁlled with RM734, a liquid crystal material that exhibits nematic (N) and ferroelectric nematic (N

) phases [

with the N–N

transition temperature

TN−NF≈133◦C

. Prior to ﬁlling, the glass channel surfaces were silanized

with hexadecyltrimethoxysilane (HDTMS) to favor planar orientation of RM734 [

]. A detailed description of the

fabrication process is given into the SI section.

Response to static ﬁelds

In the absence of an applied voltage difference

∆V

between the electrodes, the nematic director exhibits partial

ordering along the channel in both the N and N

phases (Fig. 1c), probably because of the residual roughness of

the channels walls. By applying a voltage

∆V

, which gives rise to a “nominal” electric ﬁeld

E0≡∆V/`

, the N

ordering becomes virtually perfect in all four channels (Fig. 1c-d) when

E0>0.25 V/mm

, while no signiﬁcant effect

is visible in the N phase for ﬁelds up to

E0≈50 V/mm

. We closely inspected the ﬁeld-induced N

alignment by

polarized transmission optical microscopy (PTOM) and found no sign of either defect lines or defect walls even in the

curved sections of the channels that bend contrary to the naïve electrode-to-electrode direction expected for

uniform dielectrics (Fig. 1e). This ﬁnding indicates that under these conditions both the nematic director

and the

polarization

are always nearly parallel to the solid surface and continuously follow the channel, as sketched in Fig.

1f. The continuity of N

polarization along the channels also implies a similar continuity of the electric ﬁeld, which

thus follows the channels rather than being directed along the paths connecting the electrodes as in homogeneous

dielectric media. This behavior is not dissimilar to what happens in a bent conductive wire between electrodes, where

the surface accumulation of free charges steers the ﬁeld along the wire, independently of its path. In N

this same effect

must be obtained with bound charges.

Superscreening and polarization control in conﬁned ferroelectric nematic liquids

Figure 1: Ferroelectric nematic ordering in microchannels. a: drawing of the buried microchannels. Yellow cylinders

represent the gold wire electrodes. b: pictures of the 4 channels considered in this work, designed so that the central

part forms an angle of 0

, 90

, 135

and 180

with respect to the x axis. c: PTOM microscope pictures between cross

polarizers in the absence (left) and in presence (right) of an electric ﬁeld of the “I” channel ﬁlled with RM734. The fully

dark appearance of the channel in the bottom-right picture indicates that in the presence of a ﬁeld, the orientation of the

N director is along the channel. d: PTOM microscope pictures between cross polarizers of the Z, L and S channels,

respectively, oriented so that the lateral portions are at 45

(top panels) or along (bottom panels) the analyzer while

∆V= 1V

is applied to the electrodes. e: enlargement of a curved portion of the Z channel at various orientations with

respect to the polarizer, as indicated. f: sketch of the continuity of the direction of the polarization

(green arrows)

within the S channel indicating that, in its central section, the polarization is pointing in the opposite direction with

respect to the nominal applied electric ﬁeld E0.

文档加载中……请稍候！
如果长时间未打开，您也可以点击刷新试试。

下载文档到电脑，查找使用更方便

10 玖币 0人已下载

立即下载

摘要：

SUPERSCREENINGANDPOLARIZATIONCONTROLINCONFINEDFERROELECTRICNEMATICLIQUIDSFedericoCaimi1,GiovanniNava1,SusannaFuschetto1,LianaLucchetti2,PetraPaiè3,RobertoOsellame4,XiChen5,NoelA.Clark5,MatthewGlaser5*,andTommasoBelliniy1*1Dept.ofMedicalBiotechnologyandTranslationalMedicine,UniversityofMilano,Italy2...

展开>> 收起<<

SUPERSCREENING AND POLARIZATION CONTROL IN CONFINED FERROELECTRIC NEMATIC LIQUIDS Federico Caimi1 Giovanni Nava1 Susanna Fuschetto1 Liana Lucchetti2 Petra Paiè3 Roberto Osellame4 Xi Chen5.pdf

共12页,预览3页

还剩页未读，继续阅读

声明：本站为文档C2C交易模式，即用户上传的文档直接被用户下载，本站只是中间服务平台，本站所有文档下载所得的收益归上传人(含作者)所有。玖贝云文库仅提供信息存储空间，仅对用户上传内容的表现方式做保护处理，对上载内容本身不做任何修改或编辑。若文档所含内容侵犯了您的版权或隐私，请立即通知玖贝云文库，我们立即给予删除！

SUPERSCREENING AND POLARIZATION CONTROL IN CONFINED FERROELECTRIC NEMATIC LIQUIDS Federico Caimi1 Giovanni Nava1 Susanna Fuschetto1 Liana Lucchetti2 Petra Paiè3 Roberto Osellame4 Xi Chen5

相关推荐

开通VIP享超值会员特权

作者详情

相关内容

热门标签

举报选择: