Introduction

Optical fibers are the primary technology for long-distance telecommunications1. They are based on the total internal reflection of light that occurs at the interface of a dielectric core with a higher refractive index (RI) layer, which is surrounded by a cladding layer with a lower RI, thus confining light propagation within the core. Commercial optical fibers are typically made from quartz to optimize performance2,3. However, traditional quartz optical fibers face limitations, including brittleness, susceptibility to fracture, difficulty in repair, and a lack of the reconfigurability required for on-demand manipulation, all stemming from the inherent mechanical properties of quartz4,5,6. These limitations restrict their adaptability in underwater communication (UWC). Consequently, there is a need for innovative fiber systems that preserve the essential optical functions of traditional fibers while offering additional flexibility, self-healing capabilities, and on-demand reconfigurability to meet the demands of challenging operating conditions.

In response to these challenges, researchers have explored the development of soft optical fibers by incorporating polymers and liquids, such as core-shell liquid-filled polymer fibers and hydrogel fibers7,8,9. While these soft fibers offer moderate flexibility in shape deformation, they require complex fabrication processes, are not easily repairable, and lack sufficient structural flexibility, e.g., optional shape deformation and topology change. Furthermore, many of these fibers are primarily used as sensors rather than for telecommunication, mainly because their structures are designed to maximize sensitivity to environmental parameters (e.g., RI, temperature, or chemical composition), which is beneficial for sensing but not required for long-distance signal transmission. The inherent similarity between these liquid-core–polymer-shell fibers and conventional all-glass fibers means that they still inherit the intrinsic limitations of solid fibers, such as fixed geometry, difficulty in repairing breakage, and relatively high optical loss, which further restrict their use in large-scale telecommunication10. In contrast, all-liquid optical fibers, formed by a continuous liquid jet of one fluid in air, show promise for greater resistance to breakage and for optical communication11. However, existing all-liquid optical fiber prototypes face significant obstacles, including difficulty in maintaining a stable liquid core without complex control systems, challenges in isolating light from the liquid stream, and air cladding, which is vulnerable to perturbations, rendering them impractical for real-world use. Therefore, there is a need to develop reconfigurable all-liquid optical fibers (RAOFs) that combine structural stability, fluidity, and telecommunication capability.

Recent studies have shown that structured liquids, formed by the self-assembly and jamming of nanoparticle surfactants (NPSs), can achieve structural stability and shape integrity. This approach provides a pathway for creating stable all-liquid fibers (ALFs) with both reconfigurability and mechanical strength12,13,14,15,16,17,18,19. The liquid interface offers an ideal platform for achieving total internal reflection due to the intrinsic refractive index difference (RID) between two different liquid media (Supplementary Fig. 1)8. While our long-term aim is to develop liquid fibers for underwater optical communication, we employ an oil-based external phase in the present study because such immiscible systems are experimentally well-established and allow precise control of interfacial and optical parameters. The RI and viscosity of the glucose–water inner phase can be conveniently tuned, providing a reliable platform to investigate interfacial self-assembly and waveguiding mechanisms.

In this work, building on the ALFs, we develop a soft, self-healing RAOF using all-liquid 3D printing with an RID of 0.083 at the interface, which enables total internal reflection and light propagation (Fig. 1a). This fiber is a promising alternative to traditional commercial optical fibers and is capable of efficient real-time communication. The fabrication of the RAOF relies on two key factors. First, the intrinsic RID at the water-oil interface supports total internal reflection of incident light, a critical prerequisite for the fiber’s functionality as an optical fiber. Second, interfacial self-assembly between oppositely charged ligands at the water-oil interface facilitates the 3D printing of ALFs, which can be produced at scale and shaped arbitrarily as required (Fig. 1b and 1c).

Fig. 1: Proof of concept for reconfigurable all-liquid optical fibers (RAOF).
Fig. 1: Proof of concept for reconfigurable all-liquid optical fibers (RAOF).
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a Schematic illustration of the all-liquid 3D printing process enabled by the interfacial self-assembly and jamming of NPSs at the oil-water interface, and the resultant RAOF with the capacity of light propagation. b A liquid tubule with a wavy shape prepared by all-liquid 3D printing. c Microscopic image of the liquid tubule, corresponding to the curvature marked in (b). d Total reflection of the light at the oil-water interface due to the RID derived from the high RI of the water phase (nw) and low RI of the oil phase (no). Upper panel: schematic illustration of the total reflection at the interface between the water core and oil cladder; lower panel: dark field image of the total reflection at the oil-water interface. e The demonstration of the light propagation alongside the interior interface of the wavy shape RAOF. Inset: zoom-in figure of the RAOF end. f Picture of the wavy shape RAOF. g Schematic illustration for the application of RAOF when coupling in the practical communication system. Scale bars: c and d, 2 mm; e and f, 1 cm.

Results

The formation of a stable liquid tubule with structural integrity and sustained total internal reflection at the core–cladding interface is a fundamental requirement for RAOFs. Total internal reflection at a liquid-liquid interface is made possible by the RID between two immiscible liquids. To meet the criteria of chemical stability, appropriate RID, and good processability, a glucose solution and silicone oil were used. To further increase RID, the glucose concentration was increased to yield a refractive index of 1.4876, while the silicone oil had an RI of 1.4046 (Supplementary Fig. 2a). As shown in Fig. 1d, Supplementary Fig. 1, and Supplementary Video 1, the total reflection of a red-light beam with a measured critical angle of θmeasure = 71.3° was found at the oil-water interface, closely matching the theoretical value of θc = 70.8°20. To evaluate whether the addition of ligands and the viscosity of the cladding oil influence the optical properties of RAOF, we systematically measured the RI of both phases under typical conditions. As shown in Supplementary Fig. 2b, the RI of the aqueous phase containing PSS and the oil phase containing N-PDMS remain nearly unchanged across the ligand concentration range used in this work (<5 wt.%). The maximum RI variation induced by ligands is less than 0.005, which is negligible compared to the RI modulation achieved by glucose (up to 0.15). In addition, control experiments were conducted with oils of different viscosities, and the results indicate that viscosity has almost no effect on the RI of the cladding phase or on the effective RI contrast of the RAOF. These results confirm that the RID in RAOF is predominantly governed by glucose concentration, while surfactants and oil viscosity contribute only marginally to the optical guiding properties.

To optimize light transmission, an appropriate light source should be used. Based on the UV-Vis spectra of the water core and oil cladding (Supplementary Fig. 2c), the broad transparency window of the system enables RAOFs to support a wide range of wavelengths. Here, red light (λ = 650 nm) was used due to its low absorption and attenuation in water. To test light transmission, a wavy-shaped RAOF was printed in a custom mold (Supplementary Fig. 3). Shielding plates were inserted at each bend to minimize the interference from leaked light. Upon coupling red light into the RAOF, light propagation along the inner interface was observed, with obvious leakage at the bends and coupling point (Fig. 1e), likely due to interfacial scattering at curved regions and the interface mismatch. The high RID at the oil-water interface supports total internal reflection over a large range of incidence angles, facilitating light propagation within the RAOFs. A clear output signal confirmed the ability of the RAOF to transmit light effectively (Supplementary Fig. 4), and the observed light path closely followed the printed geometry (Fig. 1f), validating light confinement within the core. To shed light on the practical application of RAOF in the real world, we coupled RAOF in a practical light communication system, and the results demonstrate that RAOF is a promising alternative to traditional commercial optical fibers in a UWC system.

Fundamentals for the preparation of reconfigurable all-liquid optical fibers

For the fabrication of the liquid tubule, poly (styrenesulfonic acid sodium) (PSS, a polyanion) was dissolved in the water phase, while NH2-PDMS-NH2 (N-PDMS, a positively charged ligand) was dissolved in the oil phase. The aqueous solution was jetted into the oil phase at a rate of 90 μL min-1 with a 0.60 mm bore needle. At the oil-water interface, the oppositely charged ligands rapidly interacted to form what we term soft nanoparticle surfactants (sNPSs) (Fig. 1a)21, as evidenced by the reduction of the interfacial tension (IFT), <10 mN m−1 (Fig. 2a). The binding energy of the sNPSs is significantly increased due to the multiple interaction sites of the PSS and the N-PDMS, stabilizing the nonequilibrium tubular shape of the liquid interface by the rapid jamming of sNPSs at the interface22. With increasing ligand content, the IFT decreases to a level insufficient to counteract gravity, causing pendant drop break-up, preventing long-term measurement. By adjusting the concentrations of PSS and N-PDMS to 3 wt.% and 3 wt.%, respectively, the jetted water evolved from semi-spherical to tubular shape (Fig. 2b), enabling all-liquid 3D printing and the preparation of the RAOFs.

Fig. 2: All-liquid 3D printing for reconfigurable all-liquid optical fibers.
Fig. 2: All-liquid 3D printing for reconfigurable all-liquid optical fibers.
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a Interfacial tension (IFT) evolution versus the mass concentration of PSS in the water phase and N-PDMS in the oil phase. b Parameter domain for the liquid tubule formation based on the amount of PSS in the water phase and N-PDMS in the oil phase. In this phase diagram, the content of PSS ([c]PSS) and N-PDMS ([c]N-PDMS) in weight percent (wt.%) was changed to alter the shape of the pendant droplet. c Various patterns fabricated by the all-liquid 3D printing, including 2D patterns, i.e., spiral, flower, a City u logo, and a 3D spring. d In-situ AFM image of the water-oil interface separating the water core and oil cladder. The lateral size is 20 μm × 20 μm, and the scale bar shows the height fluctuation of the interface. e Microscopic picture of the RAOF after settling for 10 minutes. The wrinkles indicate the structural relaxation of the interfacial assemblies. Scale bars, c 1 cm; e 500 μm.

The numerical aperture (NA), which defines the acceptance angles of the RAOFs, was calculated as NA = 0.4923, offering flexibility to couple the RAOFs into an internet system. To further benefit the fabrication of RAOFs and make them adaptable to practical applications, the parameter domain of all-liquid 3D printing for RAOFs was studied. By varying the concentration of interfacially active components, printing speed, nozzle diameter, and liquid flow rates, diameters of RAOFs ranging from ~300 µm to 1.7 mm were successfully fabricated (Supplementary Fig. 5, Supplementary Note 1). The accessible range is mainly limited by nozzle geometry, liquid viscosity, and IFT, as well as the tendency of fibers to break up due to Plateau–Rayleigh instability; this is mitigated by interfacial jamming induced by interfacial assembly, which mechanically stabilizes the fibers and preserves their tubular geometry18. The stability of RAOFs arises from the combined effects of interfacial elasticity, tension, capillarity, and viscous damping of the continuous phase. Even with a density difference between the core and oil, fibers smaller than a critical radius maintain a circular cross-section. High continuous-phase viscosity and nanoparticle-stabilized interfaces further enhance stability, enabling both two- and three-dimensional fiber designs. Quantitative estimates of these factors provide guidance for designing robust liquid fibers under gravity (Supplementary Note 2, Supplementary Fig. 6).

With the flexible all-liquid 3D printing method, we fabricated complex-RAOF patterns that can be reconfigured (Fig. 2c). However, the microscopic structure of the oil-water interface shows roughness in the sNPSs at the nanoscale (Fig. 2d), and the microscale, due to imperfections in the assemblies (Supplementary Fig. 7, Fig. 2e.). As a result, light scattering and leakage occur and contribute to the visibility of the light propagation (Fig. 1e). Additional leakage, as would be expected, was seen at high-curvature bends where the incidence angle of the light exceeded the critical angle (also seen with commercial optical fibers) (Supplementary Fig. 8). The adsorption and jamming of nanoparticles increase interfacial roughness (Sa ~ 17 nm), which enhances light scattering and optical loss compared to smooth quartz fibers; therefore, interfacial engineering to reduce roughness is essential for achieving low-loss all-liquid optical fibers. Interfacial wrinkles, resulting from the relaxation of the assemblies, significantly increase leakage and attenuate transmitted light, making their elimination essential for optimizing fiber performance. To mitigate wrinkle formation, the oil phase was initially replaced with a high-viscosity silicone oil (60,000 cSt) that slowed their development. As shown in Supplementary Fig. 9a and 9b, imperfections, such as seams on the surface of the RAOF, caused wrinkles to form normally to the axis of the RAOF24. These wrinkles were attributed to over-saturated interfacial assemblies that produced in-plane stress. We then introduced an interfacial relaxation strategy by reducing the glucose concentration in the aqueous phase, which increased interfacial fluidity and effectively eliminated wrinkles, thereby suppressing scattering and markedly improving transparency (Supplementary Fig. 9c, d)25. Interestingly, a systematic investigation revealed that the output light intensity does not follow a simple, monotonic dependence on glucose concentration. This behavior originates from a trade-off between two competing effects: (i) reduced glucose concentration lowers the refractive index difference with the oil phase, weakening total internal reflection and thus decreasing transmission, while (ii) the lower viscosity enhances interfacial relaxation, eliminating wrinkles and reducing scattering. As a result, at an optimal glucose concentration of ~60 wt.%, these competing factors are balanced, yielding wrinkle-free RAOFs and an enhancement of transmitted intensity (Supplementary Fig. 10). This trade-off provides valuable guidance for tuning interfacial morphology and optimizing RAOF performance. The attenuation of the printed RAOF was determined using the cut-back method, yielding ~0.631 dB cm−1 (Supplementary Fig. 11a). To provide a qualitative analysis of the signal and offer visual information on digital transmission, which serves as a first-order approximation of signal-to-noise ratio, clock timing jitter, and skew, the system’s eye diagram was tested (Supplementary Note 3)26. Eye diagrams measured at multiple positions along the same fiber show a reduction in eye-opening from 0.0387 V to 0.0243 V (Supplementary Fig. 11b, c), consistent with the decrease in received optical power and signal-to-noise ratio with increasing fiber length, while other metrics such as timing jitter and eye width remain nearly unchanged. Although the loss is higher than in conventional silica fibers, it is comparable to recently reported polymer-based fibers. Importantly, RAOFs can be printed in situ, repaired, and dynamically reconfigured, enabling versatile local interconnects. These results demonstrate that Gbps-level data transmission is achievable over short distances, confirming the practical potential of RAOFs for adaptive and reconfigurable optical communication networks.

Real-time communication by reconfigurable all-liquid optical fibers

The propagation of light along the RAOF enables its use in real-time communication systems. To evaluate the feasibility of the RAOF for telecommunication, it was integrated into an Ethernet platform, replacing traditional commercial optical fibers (Fig. 3a and Supplementary Fig. 12). And we used eye diagram to reveal the evolution of the communication performance at data rates ranging from 200 Mbps to 1 Gbps (Fig. 3b, c, Supplementary Fig. 13). As the data rate increases, the width and height of the eye diagram diminish, indicating a decrease in the signal-to-noise ratio. Nevertheless, the stable operation at 1 Gbps verified the feasibility of using the RAOF, demonstrating its capability to meet the basic requirements of most communication scenarios. In our current setup, the achievable data rate and bandwidth are primarily constrained by hardware factors such as the intrinsic bandwidth of the receiver and coupling efficiency. For example, by replacing the APD with a photodiode (PD), we increased the receiver bandwidth and achieved 1.28 Gbps transmission with a clearer eye diagram (Supplementary Fig. 14). This indicates that further optimization of system hardware could effectively enhance performance and enable future advancements in this technology. To further demonstrate the performance of the RAOF in a practical optical communication system, we used the Ethernet platform operating at 100 Mbps to showcase the practical use of RAOF for information transmission. Data communication speeds ranging from 1 kbps to 10 Mbps were easily achieved (Supplementary Figs. 15, 16), indicating that the RAOF can efficiently transmit light for data communication. With a certain input signal (Supplementary Fig. 17a) when the receiver was shifted from the free space optical path by a distance ( ~ 5.5 mm) and an angle ( ~ 34°)(Supplementary Fig. 17b), the loss of light causes a loss of communication signal (Supplementary Fig. 17c). However, by printing an arc-shaped RAOF to reconnect the transmitter and receiver (Supplementary Fig. 17d), the signal was restored without signal loss (Supplementary Fig. 17e), demonstrating system flexibility and ability to operate on demand.

Fig. 3: The demonstration of real-time communication with reconfigurable all-liquid optical fibers as an information transmission medium.
Fig. 3: The demonstration of real-time communication with reconfigurable all-liquid optical fibers as an information transmission medium.
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a Schematic illustration of the overall device setup of the real-time communication system. b, c Eye diagram for the real-time communication system at 200 and 800 Mbps, respectively, in which the RAOF is used as an information transmission medium. d The profile shows the source signal coupled to the light wave and the decoupled signal at the output end, which corresponds to the signal tested in (e). e Eye diagram generated from the data corresponding to the data received in (d). f A schematic illustration showing the RAOF can be used in real-time communication, such as online video meetings, corresponding to Supplementary Video 3.

High bandwidth is essential for practical applications, such as high-definition television (HDTV) streaming, which requires speeds of 5-8 Mbps. Based on our previous results, this requirement can be easily achieved using RAOFs. Motivated by this potential, we integrated RAOFs into an HDTV streaming setup. The 94/97-Mbps up/download speeds capped by our network equipment and protocols, and 25-ms network latency floored by the distance to the speed test center (Supplementary Fig. 18), indicate that the liquid fiber did not bring a significant performance penalty and could fully support HDTV streaming. Furthermore, the signal received at the output end matched the source signal (Fig. 3d), validating its suitability for real-time communication. The corresponding eye diagram also indicates the consistency between the source and receiver signals (Fig. 3e, Supplementary Fig. 19). Internet browsing using the RAOF demonstrated seamless performance with no noticeable delay (Supplementary Video 2). Finally, we showcased the potential of RAOFs for a live online video meeting. By integrating the RAOF into the communication system, we successfully established a stable connection between different countries, supporting HDTV streaming (Fig. 3f, Supplementary Video 3). These results demonstrate that the RAOF can be used as an alternative for optical fibers in practical applications, including high-speed internet and online video conferencing.

Liquid nature of reconfigurable all-liquid optical fibers

Quartz optical fibers are inherently brittle, making them prone to fracture. This brittleness often requires the replacement of damaged fibers, as repair techniques such as fusion splicing are not always practical in UWC27,28. And the coupling between fixed attachments and the quartz optical fibers also imposes limits on the practical application in underwater environments. In contrast, RAOFs, due to their liquid nature, offer significant advantages in terms of flexibility, ease of repair, structural reconfiguration, and the accessible coupling between fixed attachments and the RAOF29,30,31. Despite being printed into a nonequilibrium shape controlled by an interfacial solid-like film, RAOF retains its liquid state, allowing for on-demand adaptation to complex environments.

One of the most important benefits of the liquid state of RAOFs is its inherent self-healing capability in water environments. When fractured, the fiber can be repeatedly repaired or even self-repair, restoring the signal link and minimizing downtime. In a practical demonstration, the RAOF was used to link two computers via a wide-area network (WAN), with the signal transmission initially functioning well (Fig. 4a, b). Upon breaking the RAOF (Fig. 4c, d), the communication link was disrupted, causing the signal to cease, halting the video stream, and resulting in a loss of synchronization (Supplementary Video 4). At the breaking point, the sNPSs rapidly self-assembled, sealed the broken ends, and light propagation through the fiber was cut.

Fig. 4: The demonstration of reconfigurable all-liquid optical fibers (RAOF) with the uniqueness of self-healing, reconfiguration, and flowable core.
Fig. 4: The demonstration of reconfigurable all-liquid optical fibers (RAOF) with the uniqueness of self-healing, reconfiguration, and flowable core.
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a–f The correspondence between the state of the RAOF and the synchronization status of the real-time online video steam during the broken and repair process of the RAOF. a the RAOF before being broken; b the source signal and the signal at the receiver before the RAOF broken; c the RAOF was cut at the middle point; d the source signal and the signal at the receiver after the fiber was broken, showing the offline internet connection; e the RAOF was reparied; f the source signal and the signal at the receiver after fiber was repaired, showing that the link was resotred. g Schematic illustration of the shape reconfiguration by all-liquid 3D printing. h The picture of an RAOF. i The picture of an RAOF with an additional channel derived from h, forming a Y-branched structure with two channels, namely CH 1 and CH 2. j The signals from the light source, CH 1, and CH 2. k Schematic illustration of the signal switch by remotely controlled ferrofluid droplet movement inside the RAOF. A RAOF with a ferrofluid droplet as switch for the signal switching: l The ferrofluid droplet is in CH 2, leading to a signal on in CH 1 while a signal off in CH 2; m The ferrofluid droplet is relocated in CH 1, leading to a signal off in CH 1 while a signal on in CH 2. n The signal profiles at CH 1 and CH 2 associating with l and m. Scale bars: a–c, h, i, l, m, 1 mm.

To repair the break, a simple welding process was conducted by injecting an additional water phase into the fractured region, reconnecting the broken ends, and restoring the signal link between the two computers (Fig. 4e-f). This easy repair is rather convenient for the UWC system, which is superior to the traditional quartz optical fibers. Although there was some increased light scattering near the break due to defects (Fig. 4e), the functions remained unaffected, and the fiber’s performance was identical to its pre-break state. Notably, the connection speed after repair shows no significant attenuation compared with that before the breakage (Supplementary Fig. 20). The video stream resumed, and the link between the computers was fully restored (Supplementary Video 4). This demonstration highlights the remarkable ability of RAOFs to self-heal and recover, making it a viable and efficient solution for maintaining real-time communication links.

Structural design of RAOF

Optical waveguide devices are integral to optical communication and computing systems, serving the essential function of manipulating light beams for separation and combination. These devices rely on various optical effects, such as the electro-optical, thermo-optical, and magneto-optical effects, to control the optical field and transfer information into the optical domain. Reconfigurable optical waveguide arrays have become increasingly significant in the fields of optical switching and the development of programmable optical computing chips, offering flexible control over optical signals32,33. However, traditional optical waveguide devices are typically formed in photonic integrated circuits, photolithographically on a 2-dimensional plane of materials like lithium niobate, silicon, or silicon nitride. Once formed, the optical waveguide’s topology becomes fixed. It is possible to achieve limited changes in waveguide topology through electrical34 or thermal35 control; however, the range of these changes remains confined to a subset of the pre-formed structures. In other words, all possible configurations are already determined during the initial fabrication, and electrical or thermal control can only transition the topology from one predefined state to another. The fluidity of the RAOF not only facilitates self-repair but also enables reconfiguration in a scalable and transformable manner, distinguishing RAOFs from conventional reconfigurable optical waveguides14,36.

One such example is an optical splitter. By using all-liquid 3D printing, the RAOF can be spatially programmed to generate a new branch from the original fiber, hence forming a new Y-Branch. The bifurcation can be established at any location along the fiber to provide a user-defined functionality. Further splitting at more points can turn RAOF paths into any desired structures, without pre-design and fabrication of a superset waveguide topologies, and, thus, with remarkable freedom (Fig. 4g). With this approach, we fabricated RAOF structures in situ to split light waves into distinct channels (denoted as CH 1 and CH 2) (Fig. 4h, i). This reconfiguration enables versatile channel switching and on-demand signal selection (Fig. 4j). By positioning the receiver at different channels, the transmitted signal can be switched seamlessly (Fig. 4j, Supplementary Figs. 21 and 22, and Supplementary Video 5), demonstrating the adaptability of the reconfigurable design. Such a structurally scalable, transformable, and reconfigurable RAOF is superior to the traditional quartz optical fibers or reconfigurable photonic integrated circuits, offering the possibility to adapt the RAOF to various complicated working conditions and satisfy on-demand shape changes.

The concept of 3D reconfigurable optical waveguide design offers a new paradigm. Not only can the shape and spatial positioning of the optical waveguide be fully reconfigured, but the RAOF can also manipulate the internal light field. For traditional optical fibers and photonic integrated circuits operation, an additional switch should be coupled into the system to control signal switching and on-off control, which requires significant effort to logically design and implement. In contrast, with RAOFs, dynamic switching between channels or toggling of the signal (on and off) was easily achieved by introducing a ferrofluid droplet into the channels (Fig. 4k). The ferrofluid droplet can be remotely controlled to relocate freely inside the RAOF, acting as a switch to block or release the light signal across the tubule (Fig. 4l, m, Supplementary Video 6). The signal can thus be controlled as needed (Fig. 4n). No persistent power control is needed to maintain the switched status, as in Mach-Zehnder interferometric switch34,35. And again, the switch or block can be located arbitrarily along the fiber. Even after multiple reconfigurations, the received signals remained consistent with the source signals (Supplementary Fig. 23), confirming the structural and functional reliability of the RAOF system. The manipulation method is not limited to magnetic forces; it can be extended to other mechanisms, such as acoustic techniques. Moreover, implanting droplets into RAOF can be done at arbitrary positions and is far more convenient than conventional optical waveguides. The implant could stimulate novel and diverse effects in and by the lightwave, for instance, polarization effect, nonlinearity, and particle manipulation. This opens new opportunities for further investigation.

In summary, we successfully fabricated a RAOF for real-time communication using all-liquid 3D printing, leveraging the unique properties of liquid interfaces. The design is based on two key principles: (1) the liquid interface as a platform for interfacial self-assembly of charged polymer ligands, enabling the fabrication of liquid tubules; and (2) the intrinsic refractive index difference at the water-oil interface, facilitating total internal reflection. The self-assembly process stabilizes the RAOF structure, achieving diameters as small as 300 µm and enabling light propagation for real-time communication at speeds up to 1 Gbps. Due to its liquid nature, the RAOF exhibits exceptional softness, self-healing capabilities, and on-demand reconfigurability, making it adaptable to complex environments and resilient under impact. Moreover, the RAOF affords the on-demand manipulation of the signal transmitting anywhere inside the fiber, offering promising potential for the future development of optical fiber for on-demand operation. This innovation offers valuable insights for future implementations in underwater optical communication, where appropriate refractive index contrast and immiscible aqueous systems could be employed.

Methods

Materials

Aminopropyl terminated polydimethylsiloxane (N-PDMS, Mn ~ 3 kg mol−1, Gelest), Poly(styrene sulfonic acid) sodium salt (PSS, M.W. 7000, Thermo Scientific), Silicone oil (1000 cSt, Aladdin, China), Silicone oil (60,000 cSt, Aladdin, China), D-(+)-Glucose (AR, Aladdin, China), DIW was obtained from Milli-Q system; Polylactic Acid filament for 3D printing of the homemade mode was used as received from 3DMart.

The preparation of a compatible water phase and oil phase

The operating basis of optical fiber is the total internal reflection of light at the interface between the fiber core and cladding. Herein, the prerequisite for realizing all liquid optical fibers is to manipulate the RID across the liquid core-cladder interface. In this work, the water phase is used as the fiber core while silicone oil is chosen as the cladding. To increase the RI of the water core, glucose is used to adjust the RI of the water core37. In detail, glucose was dissolved in water with various weight percentages, as shown in Supplementary Fig. 2a, and to optimize the RID, the 77.8 wt.% group was used as the water core.

Another precondition is to realize the interfacial self-assembly to obtain a structured liquid, which can be used to prepare ALFs38. Here, PSS, a negatively charged polyanion, and N-PDMS, a positively charged ligand upon protonation, were dissolved in the water phase and oil phase, respectively. Due to their oppositely charged state, they will assemble at the oil-water interface through electrostatic interaction, forming NPS. To study the influence of ligand content on the IFT, the content of PSS and N-PDMS was adjusted, i.e., the PSS content was varied from 1.1 wt.% to 5 wt.%; N-PDMS content in oil phase was varied from 2 wt.% to 4 wt.%. The IFT was measured using a typical pendant drop method with a tensiometer39, in which a droplet of the water phase was suspended in the oil phase and imaged to extract the interfacial curvature.

3D printing of the all-liquid optical fiber

A commercially available 3D printer (Snapmaker A350T) is used to print the water solution (ink) in the oil solution (bath), which is demonstrated in Fig. 1a. For suitable printing, the ink contains [glucose]=77.8 wt.% and [PSS] = 3 wt.%, while the bath contains the [N-PDMS] = 3 wt.%.

In-situ atomic force microscopy

The in-situ AFM imaging was carried out using a typical protocol40. First, the water phase and the oil phase are synthesized according to the suitable component for 3D printing, i.e., water phase contains [glucose] = 77.8 wt.% and [PSS] = 3 wt.%, oil phase contains [N-PDMS] = 3 wt.%. Silicon wafers with <100> orientation were cut into square pieces 0.5 cm × 0.5 cm and later glued to a magnetic puck using epoxy, forming the substrates. All components, including the substrates, AFM tip holder, and AFM tips (Biolever mini AC40TS, Olympus), were cleaned in a UV Ozone tool for 10 minutes. A tiny drop (~100 nL) of the oil is dipped onto the substrate and subsequently subjected to degassing by using a vacuum chamber. Then the sample was placed on the sample stage of the Cypher ES scanner (Oxford Instruments). In the meantime, ~100 μL water phase was added to fully cover the oil droplet. Notably, due to the high viscosity of both water and oil phases, the density mismatch between oil and water phases is neglected, and the oil droplet does not float up during the experiment. In this manner, the water-oil interface was formed and was ready for characterization. The in-situ AFM imaging experiments were carried out in AC mode, and the scan range was set to 20 × 20 μm in lateral size. Before the scanning, the interface was settled for 30 minutes to allow the interfacial self-assembly at the oil-water interface.

Real-time communication by RAOF

To assess the performance of liquid optical fiber in a practical communication system, we have developed a 100 Mbps Ethernet platform. As shown in Fig. 3a, we use a personal computer (PC, DELL workstation) as the network receiver, connecting to the Internet via a 100 Mbps intensity modulation-direct detection (IM-DD) fiber optic link.

In the downlink of the Ethernet network, an RJ45 twisted pair cable connects the Internet to optical transceiver 1 (LNK-M3011-SFP). This transceiver converts the Ethernet-protocol signal into a gigabit transceiver (GTX) protocol optical signal that can be transmitted over the fiber optic link. The core components of the fiber optic link are a semiconductor laser (QSI, QL68J6SC) and an AC-coupled drive circuit on the transmitting side41, and an avalanche photodiode (APD, Thorlabs APD430A2/M) on the receiving side. The driver board supplies bias current and amplified AC signals to the semiconductor laser. The semiconductor laser emits light at a central wavelength of 650 nm, which passes through a plastic optical fiber (POF/L1 FC-FC) into the core of the liquid fiber. The plastic optical fiber has a core diameter of 1000 µm and a large numerical aperture, which allows for better coupling with the liquid optical fiber. The light is transmitted through total reflection in the liquid fiber. At the receiving end, the light from the liquid fiber is coupled into the plastic optical fiber, which is connected to the APD. Operating in the visible light band, the APD converts the received optical signal into an electrical signal for transmission to the receiving side of optical transceiver 2 (LNK-M3011-SFP). To address the power mismatch among the optical transceiver module, the input signal of the driver board, as well as the output signal levels of the APD, we added a tunable gain amplifier between the two optical transceivers and the fiber optic link. This amplifier module adjusts the input and output power levels of the fiber optic link. The tunable gain amplifier, consisting of an electrical attenuator (RBS-70-3-2) and a fixed-gain electrical amplifier (MPA-10-40), provides a gain adjustment range of −30 dB to 40 dB. To monitor the received signal of the downlink, we introduce a branch to an oscilloscope through a 1×2 power splitter (RS2DC60-S) placed between the APD and optical transceiver 2.

In the uplink, we use coaxial cables (T1-SMAMAMAM-1m) to connect the transmitting end of optical transceiver 2 to the receiving end of optical transceiver 1. By downloading data from the Internet through the downlink, we demonstrated that real-time data transmission is achievable over the liquid fiber link. To the best of our knowledge, this is the first real-time Ethernet system based on pure liquid fibers to support standard Ethernet services, including web browsing, video downloading, and online web conferencing.

Measurements of eye patterns

To evaluate the performance of the optical fiber communication system, we utilized a high-speed arbitrary waveform generator (AWG, Keysight M8190A) and a sampling oscilloscope (Agilent 86116 C Opt. 040) to measure the eye diagrams across various data rates. The AWG generated a pseudo-random sequence modulated using on-off keying with non-return-to-zero (OOK-NRZ), which was subsequently loaded onto the laser source after amplification by the tunable gain amplifier and transmitted through the optical fiber link. Considering the bandwidth limitation of the APD at 400 MHz, a higher-bandwidth PIN-type photodiode (PD, 818-BB-21A, with a 1.2 GHz bandwidth) was employed to provide enhanced system performance evaluation. Eye diagram measurements were conducted across data rates ranging from 0.2 to 1 Gbps, with an increment step of 0.2 Gbps. The experimental results indicate that, at rates below 600 Mbps, the eye diagrams are clear, with minimal signal distortion. However, at higher data rates, slight signal jitter was observed, leading to minor degradation in signal integrity.

These findings demonstrate that the optical communication system, utilizing pure liquid fiber, has shown preliminary communication capabilities and is compatible with current optical networks. This indicates considerable potential for its integration into future reconfigurable optical communication infrastructures.

Note

  1. 1.

    The world map was generated by an open-source webpage pixelmap: https://pixelmap.amcharts.com/#

  2. 2.

    The practical broadband speed for internet surfing (Supplementary Figs. 18 and 20) was tested by an open-source web tool: https://www.speedtest.net/

  3. 3.

    The authors affirm that human research participants provided informed consent for publication of the images in Supplementary Fig. 22 and Supplementary Video 3.