Reconfigurable Metasurface Antenna Based on the Liquid Metal

micromachines

Article Reconﬁgurable Metasurface Antenna Based on the Liquid Metal for Flexible Scattering Fields Manipulation

Ting Qian

Shanghai Technical Institute of Electronics and Information, Shanghai 200240, China; [email protected]

Abstract: In this paper, we propose a reconfigurable metasurface antenna for flexible scattering field manipulation using liquid metal. Since the Eutectic gallium indium (EGaIn) liquid metal has a melting temperature around the general room temperature (about 30 ◦C), the structure based on the liquid metal can be easily reconstructed under the temperature control. We have designed an element cavity structure to contain liquid metal for its flexible shape-reconstruction. By melting and rotating the element structure, the shape of liquid metal can be altered, resulting in the distinct reflective phase responses. By arranging different metal structure distribution, we show that the scattering fields generated by the surface have diverse versions including single-beam, dual-beam, and so on. The experimental results have good consistency with the simulation design, which demonstrated our works. The presented reconfigurable scheme may promote more interest in various antenna designs on 5G and intelligent applications.

Keywords: liquid-metal metasurface; reconﬁgurable metasurface; reconﬁgurable antenna; beam manipulation

Citation: Qian, T. Reconfigurable 1. Introduction Metasurface Antenna Based on the The concept of metamaterials has attracted much attention in the past decade. Meta- Liquid Metal for Flexible Scattering materials are three-dimensional artificial structures with special electromagnetic properties. Fields Manipulation. Micromachines Due to the fact that metamaterials can be designed artificially, they can be widely used in a 2021, 12, 243. https://doi.org/ variety of applications, such as negative and zero refraction [1], perfect absorption [2–4], 10.3390/mi12030243 invisibility cloaking [5–8], dielectrics lenses [9,10] and vortex beams [11,12]. Reconfigurable metamaterials are composed of passive metamaterials and active components [13]. Its Academic Editor: Daeyoung Kim rapid development makes it possible to manufacture metadevices with practical functions and unique subwavelength devices [14–19]. At present, the tuning methods of reconfig- Received: 1 February 2021 urable metamaterials mainly include mechanical tuning [20,21], electronic tuning [22–26], Accepted: 25 February 2021 Published: 28 February 2021 material property tuning [27–29] and optical tuning [30,31]. The antenna is an essential part of the wireless information system. With the rapid

Publisher’s Note: MDPI stays neutral development of wireless information technology, higher and higher requirements are put with regard to jurisdictional claims in forward for the antenna system. It is expected that the antenna unit has the characteris- published maps and institutional affil- tics of a wide beam, wide frequency band and high gain. Therefore, people continue to iations. explore new concepts and technologies of various antennas to break through the related problems. In recent years, the concept of reconfigurable antenna has been proposed. The reconfiguration of antenna performance is realized by adjusting the state of controllable devices integrated in the general radiation aperture. There are three methods to realize reconfiguration of antenna: electronic device reconfiguration (such as pin diode [32,33], Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. radio frequency (RF) switches [34–36], varactor diode [37–39], etc.), mechanical reconfig- This article is an open access article uration [40,41] and changing the material properties of the antenna [42]. Through the distributed under the terms and reconfigurable technologies, the performance of antenna can be transformed, such as conditions of the Creative Commons the working frequency [43–45], polarization mode [46–48], radiation pattern [49] and the Attribution (CC BY) license (https:// combination of the above performance. creativecommons.org/licenses/by/ However, many of the researches we mentioned above need complex hardware system 4.0/). design, requiring active components like PIN diodes, switches, and varactors, demanding

Micromachines 2021, 12, 243. https://doi.org/10.3390/mi12030243 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, x 2 of 8

However, many of the researches we mentioned above need complex hardware sys- Micromachines 2021, 12, 243 tem design, requiring active components like PIN diodes, switches, and varactors,2 de- of 8 manding high control complexity and cost. To explore a flexible and low-cost reconfigurable method, here, we present a metasurface antenna based on liquid metal for flexible scatteringhigh control fields complexity manipulation. and cost. ATo square-rin explore ag flexiblecavity andstructure low-cost has reconfigurable been designed, method, simu- latedhere, weand present fabricated a metasurface to demonstrate antenna our baseddesign on. By liquid rotating metal and for reco flexiblenstructing scattering the liquid fields metalmanipulation. shape, the A square-ringreflected phase cavity response structure of has the been element designed, shows simulated various distinct and fabricated states, coveringto demonstrate more than our design.270° phase By rotatingrange. We and designed reconstructing several thephase liquid distributions metal shape, on the surfacereflected to phase generate response the scattering of the element beam in shows different various directions. distinct The states, measured covering results more have than good270◦ phase agreement range. with We the designed simulation, several verifying phase distributionsour design. We on believe the surface this work to generate will stim- the ulatescattering more beamresearches in different on reconfigurable directions. The and measured flexible designs results utilizing have good liquid agreement metal for with di- versethe simulation, applications. verifying our design. We believe this work will stimulate more researches on reconfigurable and flexible designs utilizing liquid metal for diverse applications. 2. Principle and Design 2. Principle and Design As shown in Figure 1, we presented a metasurface antenna system composed of a reflectedAs shown surface in with Figure reconfigurable1, we presented liquid a metasurfacemetal on it and antenna a feed system antenna. composed When the of in- a cidentreflected wave surface from withthe feed reconfigurable antenna is reflected liquid metal by the on specific it and metal a feed structure, antenna. the When desired the phaseincident distribution wave from is the generated, feed antenna as illustrate is reflectedd in by Figure the specific 1. According metal structure, to the length the desired from thephase phase distribution center of is the generated, antenna asto illustratedthe different in Figureelements,1. According the relevant to the phase length distribution from the onphase the centersurface of is the obtained antenna easily to the according different to elements, the equation the relevant below, as phase we exhibited distribution in Figure on the 1b.surface is obtained easily according to the equation below, as we exhibited in Figure1b. 2π2𝜋LL(m,m, n n) Phase𝑃ℎ𝑎𝑠𝑒(m,m, n) n= = (1) in λ𝜆 wherewhere L(m, L(m, n) means n) means the lengththe length from from the phasethe phase center center of feed of feed antenna antenna to theto the element element as asmth m rowth row and andnth n column,th column,λ is 𝜆 the is the wavelength wavelength of the of the operating operating frequency. frequency. To designTo design the thedesired desired scattering scattering pattern, pattern, we we first first calculate calculate the the phase phase distribution distributionPhase 𝑃ℎ𝑎𝑠𝑒de ofthe of the specific spe- cificbeam beam scattering scattering fields, fields, such assuch beam-deflection as beam-deflection or dual-beam or dual-beam fields. fields. Then theThen final the phase final phasedistribution distribution can be can calculated be calculated following following Equation equation (2) as below.(2) as below.

Phase f inal = Phasede − Phasein (2) 𝑃ℎ𝑎𝑠𝑒 =𝑃ℎ𝑎𝑠𝑒 −𝑃ℎ𝑎𝑠𝑒 (2)

Figure 1. TheThe schematic schematic of of the the presented presented reconfigurable reconﬁgurable metasurface metasurface antenna. antenna. (a ()a The) The anomalous anomalous refraction under plane-wave incidence. ( b) The incident phase calculation of the point source feed.feed.

◦ ◦ ◦ To simplifysimplify thethe phase phase states, states, we we designed designed the th 2-bite 2-bit reflective reflective states states as 0 as, 90 0°,, 90°, 180 ,180°, and ◦ and270 270°to establish to establish the finalthe final surface surface phase phase response. response. The finalThe phasefinal phase distribution distribution is generated is gen- eratedby liquid-metal by liquid-metal elements elements with differentwith differ shapes,ent shapes, which which can be can achieved be achieved by heating by heating and andcooling cooling at a certainat a certain temperature. temperature. By arranging By arranging the various the various phase phase distributions distributions produced pro- ducedby the by liquid-metal the liquid-metal elements, elements, we can we realize can re variousalize various radiation radiation patterns patterns like single-beam, like single- beam,dual-beam, dual-beam, and three-beam. and three-beam. To achieve the flexibleflexible phase-reconfigurablephase-reconfigurable capability, wewe designeddesigned aa metasurfacemetasurface element with a rectangular-ring cavity to co containntain the liquid metal and guide it into the desired shape. The detailed element structure is presented in Figure2a. In the upper layer, the cavity structure of the element is polyamides within three-dimensional printing technology, whose permittivity is 3. Eutectic gallium indium (EGaIn) alloy with a melting point around room-temperature is employed for the desired shape reconstruction. The Micromachines 2021, 12, x 3 of 8

desired shape. The detailed element structure is presented in Figure 2a. In the upper layer, the cavity structure of the element is polyamides within three-dimensional printing tech- Micromachines 2021, 12, 243 3 of 8 nology, whose permittivity is 3. Eutectic gallium indium (EGaIn) alloy with a melting point around room-temperature is employed for the desired shape reconstruction. The designed dimension of the element structure is provided as follows: a = 12 mm, b = 5mm, c+ddesigned = 12 mm, dimension h1 = 2.5 of mm, the elementh2 = 1.6 structuremm. The isalloy provided shape asreconstruction follows: a = 12 method mm, b was = 5 mmfirst, meltedc + d = 12under mm, a h1heating = 2.5 mm,temperature h2 = 1.6 a mm. little The high alloyer than shape its reconstructionmelting point. methodThen we was rotated first themelted element under at aa heatingcertain angle temperature to make a the little liquid higher metal than flow its melting into the point. specific Then size, we as rotatedshown inthe Figure element 2b. at After a certain the alloy angle reaches to make the the target liquid dimension, metal flow we into cool the the specific metal to size, fix asits shownshape. Toin Figuresimplify2b. the After control the alloy states reaches of liquid the targetmetal, dimension, we design wefour cool states the with metal the to fixliquid its shape. metal Tolength simplify as follows: the control (1) d states= 6.6 mm, of liquid (2) d metal,= 9.2 mm, we design(3) d = four8.04 mm, states (4) with d = the7.65 liquid mm. metalThese fourlength states as follows: relate to (1) the d =2-bit 6.6 mm,phase (2) responses d = 9.2 mm, as 0°, (3) 90°, d = 8.04180°, mm, and(4) 270°, d = theoretically. 7.65 mm. These To ◦ ◦ ◦ ◦ simplyfour states express relate the to phase the 2-bit states phase of these responses four re assponses, 0 , 90 , we 180 encode, and 270them, theoretically.as 0, 1, 2, and To 3, respectively.simply express Figure the phase 2c,d list states the of simulated these four phase responses, and amplitude we encode response them as for 0, 1, reflection 2, and 3, wave.respectively. The element Figure simulations2c,d list the are simulated performed phase in a and commercial amplitude software, response CST for microwave reflection wave.studio. The The element periodic simulations boundary condition are performed and linear in a commercial polarization software, incidence CST are microwave applied in thestudio. simulations. The periodic From boundary the curve condition data, we andcan clearly linear polarizationobserve that incidencethe reflected are appliedamplitude in responsesthe simulations. of four From states the are curve all above data, we−3 dB, can promising clearly observe a fair that reflection the reflected efficiency amplitude under planeresponses wave of incidence. four states It areshould all above be noted−3 dB,that promising the reflection a fair efficiency reflection can efficiency be further under im- planeproved wave by applying incidence. the Itmaterial should with be noted lower that loss. the The reflection phase responses efficiency are can provided be further in Figureimproved 2c, byin which applying the thefour material states are with labele lowerd in loss. different The phase colors, responses respectively. are provided The simu- in latedFigure four2c, in states which exhibit the four 170.2°, states 81.8°, are labeled −11.4°, in − different101.1° at colors,8 GHz, respectively. whose phase The di simulatedfference is four states exhibit 170.2◦, 81.8◦, −11.4◦, −101.1◦ at 8 GHz, whose phase difference is about about◦ 90°. To indicate the phase arrangement on the reflecting surface clearly, we have used90 . Tothe indicate phase code the phase “0”, “1”, arrangement “2”, and “3” on theto represent reflecting these surface four clearly, phase westates. have used the phase code “0”, “1”, “2”, and “3” to represent these four phase states.

Figure 2. TheThe element element design design and and its its simulation simulation performance. performance. (a) The (a) Thedetailed detailed structure structure of the of the metasurface element. (b) The liquid-metal reconstructionreconstruction byby rotatingrotating thethe element.element. ((cc)) TheThe simulatedsimu- latedresults results of reﬂection of reflection phase responsesphase responses of four of element four element dimensions, dimensions, where d where is 6.6 mm,d is 6.6 9.2 mm, 8.049.2mm, mm, 8.047.65 mm, mm. 7.65 (d) Themm. simulated (d) The simulated results of results reﬂection of reflection amplitude amplitude responses responses of four element of four element dimensions, wheredimensions, d is 6.6 where mm, 9.2d is mm, 6.6 mm, 8.04 9.2 mm, mm, and 8.04 7.65 mm, mm. and 7.65 mm.

The architecture of the presented metasurface antenna is already shown in Figure 1. A metasurface array composed of 15 × 15 elements is designed and simulated in CST. Two feed source schemes are applied to demonstrate the scattering field manipulation performance. One is plane-wave excitation while the other applies micro-strip antenna as feed, as showed in Figure 1a,b, where the focal length of the antenna feed is set as 60mm. For Micromachines 2021, 12, 243 4 of 8 the plane wave incidence, we design three phase distributions to realize beam-deflection, dual-beam, and trinal-beam, respectively, as listed in Figure 3a–c. For the point source incidence, we designed a phase distribution, as shown in Figure 3d, to obtain a nearly consistentbeam-deflection, phase distribution dual-beam, andon the trinal-beam, reflection respectively, surface. To asdemonstrate listed in Figure their3a–c. performance, For wethe performed point source the incidence, full-wave we simulation designed ain phase CST to distribution, test the scattering as shown fields. in Figure The3 d,related to far- obtain a nearly consistent phase distribution on the reflection surface. To demonstrate fields simulation data is presented in Figure 3e–h, in which the open boundary condition their performance, we performed the full-wave simulation in CST to test the scattering is fields.applied The in related all far-fieldsschemes. simulation In Figure data 3a–c, is presented four phase in Figure distributions3e–h, in which are the designed open as “0001112223333”,boundary condition “000222”, is applied and in all“012300010003210”. schemes. In Figure3 Ina–c, Figure four phase 3a, the distributions single beam are scatter- ingdesigned direction as “0001112223333”,is at about 20° based “000222”, on the and gradient “012300010003210”. phase distribution. In Figure 3Fora, the the single dual-beam scatteringbeam scattering field, directionthe two ismain at about beams 20◦ based poin ont at the about gradient 30° phase when distribution. applying Forphase the code ◦ “000222”.dual-beam As scattering the designed field, metasurface the two main has beams a 15 point × 15 at unit about array, 30 whenthe code applying “000222” phase fills the arraycode as “000222”. “00022200022200” As the designed along metasurface the x-axis haswhil ae 15 keeping× 15 unit the array, same the phase code state “000222” along the otherfills thedirection array as(y-axis). “00022200022200” Other phase along codes the fox-axisllow whilethe same keeping arranging the same method. phase stateThe phase along the other direction (y-axis). Other phase codes follow the same arranging method. distributions in Figure 3c,d are the same, but their excitation feeds are different. Figure 3g The phase distributions in Figure3c,d are the same, but their excitation feeds are different. presentsFigure3 gthe presents far-field the result far-field under result plane-wave under plane-wave illumination, illumination, where where a three-beam a three-beam scattering fieldscattering is clearly field observed. is clearly observed. Figure 3h Figure presents3h presents the far-field the far-field result result under under micro-strip micro-strip antenna feeding,antenna in feeding, which inthe which beam the gain beam is gainabout is 10 about dB. 10 dB.

FigureFigure 3. The 3. The designed designed metasurface metasurface schemesschemes for for different different beam beam manipulation manipulation and the and related the simulationrelated simulation results for far-fields.results for far- fields.(a– (da)– Thed) The metasurface metasurface elements elements configuration configuration for different for different surface phasesurface distributions, phase distributions, in which the in phasewhich code the alongphase code alongx-axis x-axis is “0001112223333”,is “0001112223333”, “012300010003210”, “012300010003210”, “000222” “000222” and “012300010003210”, and “012300010003210”, respectively. respectively. Please note Please that the note phase that the phasedistributions distributions are are the samethe same in (b ,ind), (b but,d), their but excitationstheir excitations are plane-wave are plane-wave and antenna-source, and antenna-source, respectively. respectively. The simulated The sim- ulatedfar-field far-field results results for the for related the related phase phase schemes schemes are listed are in listed (e–h), wherein (e–h (e),– gwhere) are excited (e–g) are by plane-wave excited by andplane-wave (h) is excited and (h) is excitedby antenna.by antenna.

In the experimental veriﬁcation, we fabricated a 15 × 15 metasurface sample to In the experimental verification, we fabricated a 15 × 15 metasurface sample to demonstrate the design we presented above. Since the metasurface is composed of two demonstratesubstrate layers, the design we fabricated we presented the bottom above. substrate Since layer the andmetasurface upper polyamides is composed layer of two substrateinto one piece.layers, The we bottom fabricated substrate, the bottom FR4, and substrate the bottom layer metal and layer upper are allpolyamides fabricated layer intotogether one piece. into one The board bottom within substrate, Printed CircuitFR4, and Board the (PCB) bottom technology. metal layer The transparentare all fabricated togetherpolyamides into containingone board liquid within metal Printed is fabricated Circuit based Board on (PCB) 3D printing technology. technology, The wheretransparent polyamidesthe rectangular-ring containing cavity liquid is sculptured metal is fabricated on the substrate. based on Each 3D cavity printing is injected technology, with a where thecertain rectangular-ring amount of melted cavity EGaIn is sculptured alloy to construct on the substrate. the designed Each metal cavity structure. is injected After with a the liquid metal solidiﬁes, we tightly glue the two dielectric plates together. It is worth mentioning that single element manipulation is possible if we can use a point heater to melt one element while keeping the surrounding elements at a low temperature. The most

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certain amount of melted EGaIn alloy to construct the designed metal structure. After the liquid metal solidifies, we tightly glue the two dielectric plates together. It is worth mentioning that single element manipulation is possible if we can use a point heater to melt Micromachines 2021, 12, 243 one element while keeping the surrounding elements at a low temperature. 5The of 8 most time-consuming part is metal shape reconstruction, which mainly depends on the melting time of the liquid metal. In our experiment, the melting time of liquid metal is about 30 seconds.time-consuming For a large part array is metal modulation, shape reconstruction, we can apply which a mainlyfast heating depends and on cooling the melting device to arrangetime of the the phase-pattern liquid metal. In distribution our experiment, more the qu meltingickly. Some time of other liquid solutions metal is about such 30as s.phase- patternFor a largeoptimization array modulation, reduce the we amount can apply of a th faste reconfigurable heating and cooling element device each to arrangetime. By ap- plyingthe phase-pattern several fast distributionheating and more cooling quickly. devices, Some we other estimate solutions that such the as melting phase-pattern time can be reducedoptimization to less reduce than 10 the seconds, amount ofand the the reconfigurable average time element of the eachmetasurface time. By applyingpattern recon- several fast heating and cooling devices, we estimate that the melting time can be reduced figuration can be limited to about three minutes. The fabricated sample picture is pro- to less than 10 s, and the average time of the metasurface pattern reconfiguration can be videdlimited in Figure to about 4a, three in which minutes. an Theenlarged fabricated elem sampleent figure picture is given. is provided For plane-wave in Figure4a excita-, tionin schemes, which an the enlarged metasurface element is figure directly is given. illuminated For plane-wave by a high-gain excitation horn schemes, at long distance, the to metasurfaceimitate an approximate is directly illuminated plane-wave by a incidence. high-gain hornFor microstrip at long distance, antenna to imitate excitation, an we fabricatedapproximate another plane-wave PCB plate incidence. to excite For the microstripmetasurface. antenna The antenna excitation, was we set fabricated at the central axisanother and about PCB plate60 mm to excite away the from metasurface. the surface. The The antenna antenna was PCB set at plate the central was fixed axis and with the liquid-metalabout 60 mm metasurface away from the using surface. polyamide The antenna scre PCBws, plate as showed was fixed in with Figure the liquid-metal4b. The far-field measurementmetasurface usingwas polyamideperformed screws, in a standard as showed microwave in Figure4b. chamber The far-field room. measurement A two-dimen- was performed in a standard microwave chamber room. A two-dimensional far-field of sional far-field of the metasurface was tested based on a plane turning table. the metasurface was tested based on a plane turning table.

Figure 4. The fabricated metasurface antenna sample photograph. (a) Photograph of the liquid-metal metasurface sample as well as its enlarged ﬁgure of the element. (b) The assembled metasurface antenna fed by a micro-strip antenna. Figure 4. The fabricated metasurface antenna sample photograph. (a) Photograph of the liquid-metal metasurface sample as well as its enlarged figure of the Theelement. measurement (b) The assembled results are metasurface given in Figure antenna5, infed which by a micro-strip we perform antenna. three phase distributions to respectively demonstrate single-beam, dual-beam, three-beam under plane- wave,The and measurement point source results excitation. are Fromgiven the in measuredFigure 5, data,in which we can we clearly perform observe three the phase distributionssingle-beam to deﬂection, respectively dual-beam, demonstrate and three-beam, single-beam, respectively, dual-beam, in Figure three-beam5a–c when under plane-wave,under plane-wave and point illumination. source excitation. For microstrip From antenna the measured feed, the data, tested we result can isclearly showed observe thein single-beam Figure5a, where deflection, the gain dual-beam, of the main beamand thr isee-beam, about 10 dB. respectively, Compared toin theFigure previous 5a–c when works [15–17], the directivity of the presented four schemes in this work is about 10% under plane-wave illumination. For microstrip antenna feed, the tested result is showed lower, resulted from the substrate loss of FR4. But this could be improved by using the in high-qualityFigure 5a, where substrate the withgain lowerof the loss. main Comparing beam is about the measured 10 dB. Compared results and to simulated the previous worksresults, [15–17], we can the observe directivity a slight of difference the presented in the scattering four schemes angle ofin the this main work beams. is about The 10% lower,scattering resulted energy from inother the substrate directions loss also showsof FR4. the But discrepancy this could between be improved simulations by using and the high-qualityexperiments. substrate The reasons with for lower these loss. differences Comparing mainly the include: measured (1) the results fabrication and errorsimulated results,in polyamides we can observe cavity support a slight and difference manual assembling,in the scattering which angle may leadof the to main the error beams. in The scatteringliquid-metal energy dimensions; in other (2)directions The non-ideal also shows plane wavethe discrepancy source from between the horn antenna;simulations (3) and experiments.The manual The operation reasons in experimental for these differences measurements; mainly (4) include: The phase (1) center the fabrication error between error in simulation and measurements. polyamides cavity support and manual assembling, which may lead to the error in liquid-

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metal dimensions; (2) The non-ideal plane wave source from the horn antenna; (3) The manual operation in experimental measurements; (4) The phase center error between sim- Micromachines 2021, 12, 243 6 of 8 ulation and measurements.

Figure 5. The measured far-field results for the designed four-phase distributions, as well as the related simulation data for comparison.Figure 5. The (a–c measured) The measured far-field far-field results results for the for designed plane-wave four-phase incidence distributions, to realize one-beam, as well as dual-beam, the related and simulation three-beam. data for comparison. (a–c) The measured far-field results for plane-wave incidence to realize one-beam, dual-beam, and three- (d) The single beam reflection under antenna excitation. beam. (d) The single beam reflection under antenna excitation. 3. Conclusions 3. Conclusions In summary, we present a reconfigurable metasurface antenna using EGaIn alloy In summary, we present a reconfigurable metasurface antenna using EGaIn alloy to to realize flexible scattering fields manipulation. By changing the metal shape of the realize flexible scattering fields manipulation. By changing the metal shape of the metasurface element along the polarization direction, the reflected element phase responses metasurface element along the polarization direction, the reflected element phase re- show distinct phase differences. We designed a rectangular-ring cavity to contain and sponses show distinct phase differences. We designed a rectangular-ring cavity to contain guide the liquid metal into the desired shape. An array of 2-bit phase responses (0◦, 90◦, and guide the liquid metal into the desired shape. An array of 2-bit phase responses (0°, 180◦, and 270◦) were achieved using four dimensions of alloy length. Since the EGaIn 90°, 180°, and 270°) were achieved using four dimensions of alloy length. Since the EGaIn alloy has a melting point of around 30 °C, the alloy is easily melted under slight heating alloy has a melting point of around 30 ℃, the alloy is easily melted under slight heating and reconstruct into the designed size. Comparing to the idea proposed in Lei Chen’s and reconstruct into the designed size. Comparing to the idea proposed in Lei Chen’s work [50], this work presents a more powerful phase-manipulation method that can work [50], this work presents a more powerful phase-manipulation method that can achieve not only 2-bit phase states but also almost continuous phase-manipulation in one singleachieve element. not only The 2-bit rectangular phase states ring but structure also almost presented continuous in this phase-manipulation paper exhibits a more in one flexiblesingle phase-manipulationelement. The rectangular capability ring structure with a large presented modulation in this range. paper Allexhibits elements a more were flex- fabricatedible phase-manipulation into one metasurface capability configuration, with a larg apparentlye modulation improving range. Al itsl elements integration were level. fab- Fourricated phase into distributions one metasurface are designed configuration, for plane-wave apparently and microstrip improving antenna its integration excitation level. to obtainFour single-beamphase distributions deflection, are dual-beam, designed for and plane-wave three-beam, and respectively. microstrip We antenna performed excitation the experimentalto obtain single-beam verification deflection, for three-phase dual-beam, distribution and three-beam, designs and respectively. obtained good We agreement performed with the simulation data. Comparing to the active reconfigurable methods such as utilizing PIN diodes and varactors, the liquid-metal metasurface antenna presented in this work has much less system complexity (no other control unit), lower cost (generally one-tenth of

other methods), and lower power consumption (no operating current). The ﬂexible and powerful manipulation control of liquid metal shows good performance and potential Micromachines 2021, 12, 243 7 of 8

in the next generation of wireless communication applications and smart scenarios for microwave scattering manipulation.

Author Contributions: Conceptualization, T.Q.; methodology, T.Q.; software, T.Q.; validation, T.Q.; formal analysis, T.Q.; investigation, T.Q.; resources, T.Q.; data curation, T.Q.; writing—original draft preparation, T.Q.; writing—review and editing, T.Q.; visualization, T.Q.; supervision, T.Q.; project administration, T.Q. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Conﬂicts of Interest: The author declares no conﬂict of interest.

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