A spring balance is a macro-scale dynamometer, which is often used by people to weigh various fruits, vegetables, commodities, etc. If the size is reduced to the micro-nano scale, is it possible to "weigh" tiny forces in a calibrated way like a spring balance?

This requires significantly reducing the detection limit of the sensor, which places extremely high requirements on the precision of the device. In previous studies, scientists often solved this problem by optimizing the mechanical properties of the structure itself.

Recently, a team from West Lake University and the Hangzhou Institute of Advanced Studies of the National University of Science and Technology of China collaborated to create a new type of optical fiber spring sensor from the perspective of optimizing the processing technology.

The elastic coefficient k of the optimal spring that can be processed in this study is reduced to 44.5 pN/nm. The sensitivity is 4 orders of magnitude higher than the results of the reported literature. The detection limit reaches 40 pN (pN, 10-12N). Comparable to most MEMS devices.

The bovine-grade optical fiber sensor is expected to open up new ideas for improving sensor accuracy and has reference significance for many disciplines at the application level. For example, scanning spring fiber optic sensors are expected to be used in fields such as thin film Young's modulus measurement and biomechanical sensing. The "non-contact" detection mode has application value in turbulence detection, optical force measurement and other fields.

The analysis and exploration of preparation aspects in this study can be applied to the processing of other complex 3D structures and improve the performance of 3D structures in other application scenarios.

In addition, precise measurement of tiny forces is one of the important references when explaining physical phenomena at the micro- and nanoscale. In this study, fiber optic sensors were also used in the field of microscopic air flow force measurement.

Thanks to the high precision of the sensor, the researchers captured the nonlinear change process of airflow force with air pressure near the air outlet during the experiment, which demonstrated the potential of high-precision mechanical sensors in sensing microphysical phenomena.

Qiu Min, Guoqiang Chair Professor of Westlake University, pointed out, "We expect that cow-level optical fiber sensors will be used in multi-disciplinary basic research and exploration. More and more physical phenomena may be discovered through mechanical sensing. In the future, through precision mechanics Measurements promise to contribute to fundamental scientific discoveries."

The reviewer commented on this study: "Qiu Min et al. proposed an interesting study that combined nanomechanics, additive manufacturing and photonics to realize a bovine-level optical fiber end face mechanical sensor. This study is a nanomechanical A good example in engineering with important fundamental research relevance and application potential.”

Recently, the relevant paper is "Fiber-Integrated Force Sensor using 3D Printed Spring-Composed Fabry-Perot Cavities with a High Precision Down to Tens of Piconewton" The title was published in Advanced Materials[1].

Shang Xinggang, a doctoral student at West Lake University, is the first author of the paper. Wang Ning, an associate researcher at the Hangzhou Institute for Advanced Study at the University of Science and Technology of China (formerly a postdoctoral fellow in Professor Qiu Min’s research group), Zhou Nanjia, a distinguished researcher at West Lake University, and Qiu Min, the Guoqiang Chair Professor at West Lake University, are Co-corresponding author of the paper.

How to measure tiny forces at the micro-nano scale?

The main difficulty in measuring tiny forces at the micro-nano scale lies in the simultaneous adaptation of sensing accuracy, versatility and usage scenarios. For example, although microelectromechanical systems, atomic force microscopes, etc. have high precision of Picoton level, they are usually developed for specific purposes. They have problems such as high price, complicated use, limited versatility, and inability to be integrated with flexible and wearable scenarios. .

On the other hand, there are many technologies for measuring small deformation, such as direct imaging, capacitive, resistive, optical ranging, etc. Among them, direct imaging measurement equipment is simple and can achieve in-situ real-time observation. However, the measurement accuracy of this technology is limited by the diffraction limit; the electrical testing method has accurate signals, extremely high sensitivity, and high detection limit, but it is extremely susceptible to electromagnetic signal interference.

Fiber-based microforce sensors have outstanding advantages such as flexibility, all-optical integration, and resistance to electromagnetic interference, providing new ideas for solving the above problems. Traditional optical fiber mechanical sensors generally use optical fiber splicing microcavities, fiber Bragg gratings, cantilever beams and other methods for mechanical sensing, and have been used in many fields.

However, considering the limitations of the mechanical sensitivity of these sensing units, the accuracy is mostly in the nanonewton level, making it impossible to carry out high-precision mechanical detection. Among them, the technical difficulty lies in the precision processing of highly sensitive micro-nano 3D structures, which greatly limits the application of optical fiber sensors in the field of high-precision mechanical detection.

Qiu Min's laboratory focuses on optical devices and technology, and has rich experience in the design and preparation of optical fiber devices. In this work, they chose to directly face the difficulties in the precision design and processing of 3D structures of optical fiber sensors, and reduced the detection limit of the sensor by optimizing the preparation process and solving the pain points in this field.

Let spring scales "show their talents" at the micro scale

The basic principle of this research is simple, using the "interference" learned in college physics. When light passes through the interface of two media, reflection and transmission occur. From this, two optical interfaces can be constructed, allowing the reflected light at the interface of the two layers to produce interference superposition.

At this time, the light of a specific wavelength "interferences destructively", and the light of a specific wavelength "interferences constructively", so an oscillation-shaped spectrum is produced. The center wavelength of the spectrum trough is closely related to the distance between the two layer interfaces.

When the spiral flat plate structure is designed to the fiber end face, the flat plate and the fiber end face naturally form the above two optical interfaces, naturally forming a Fabry-Perot (FP) cavity. In the experiment, the researchers used the drift of the analytical spectrum to obtain the structural stress with the help of the intermediate value of the structural compression.

This topic originated from a research discussion between Wang Ning and Shang Xinggang. They proposed, "Since macroscopic spring balances are so common in daily life, we also want to make them show their talents at the microscopic scale." After literature research, they found that there have been some studies Researchers are paying attention to the concept of micro-nano spring scales, but the reported device performance is far lower than other types of sensors.

In order to solve the problem of insufficient accuracy of optical fiber sensors, the cross-cutting topics of optics, micro-nano mechanics, micro-nano additive manufacturing and fluid mechanics have gradually become clear.

Peeling off cocoons layer by layer: realizing high-performance 3D structured micro-nano springs

After the team's preliminary discussions and experimental exploration, the researchers faced two most critical challenges: optimization of the preparation process and calibration of device performance.

In terms of optimizing the preparation process, they experimentally chose two-photon polymerization 3D processing to prepare the spring structure. However, realizing high-performance 3D structural micro-nano springs is a great challenge. If you want to obtain better detection limits, you must meet the stringent requirements for spring geometric parameters to the greatest extent.

In conventional processing processes, post-processing processes such as development and cleaning often cause structural collapse. To this end, after reading a large amount of literature on 3D micro-nano processing, the researchers found the core cause of structural collapse—capillary force.

Subsequently, after more than a year of process exploration, they started from the principle and process of capillary force generation, improved the mechanical model of structural stress when liquid volatilizes [2], and finally adopted a low surface tension cleaning agent solution.

Although the spring structure had existed in the original design drawings for a long time, the members of the research team were still shocked when they saw the structurally complete and stable spring under the electron microscope for the first time. Qiu Min said, "This not only demonstrates the feasibility of the entire design route, but also affirms our efforts in processing optimization for more than a year."

In addition, the calibration of device performance is also a big problem. Commonly used measurement systems are difficult to adapt to the scale and mechanical properties of micro-nano springs, so they adopted micron particle calibration.

"We can compare micro-nano springs to macroscopic spring balances. At this time, micron particles play the role of weights in the macroscopic scale. By testing particles of different particle sizes, we obtained the force curve of the spring, and then analyzed and obtained the sensitivity of the device. and detection limit." Shang Xinggang said.

With the help of Dr. Hehao Chen from Nanjia Zhou's laboratory, the researchers suddenly had an idea when using microneedles to transfer micron particles: What would happen if the airflow at the needles was applied to the surface of the structure?

After further exploration, they discovered an "unexpected gain". Not only can the airflow force be sensed by the sensor, there is also a non-linear changing trend between the airflow force and the air pressure.

"After we brought the data to Professor Fan Dixia of West Lake University, we began to try to explain the experimental phenomenon through simulation. In the end, the computational fluid dynamics simulation results obtained were consistent with the experiment. This experiment well proved the advantages of high-precision sensors ." Wang Ning said.

Will continue to expand the application scenarios of spring sensors

Professor Qiu Min's laboratory at West Lake University has long been conducting basic and applied basic research in interdisciplinary fields, including optoelectronics, materials, energy, machinery, chemistry, etc. The main research directions are advanced micro-nano processing technology, micro-nano photon theory and optoelectronic devices, and key optics Theory and technology, etc.

Previously, the laboratory has conducted systematic research on the mechanical properties of micro-nano springs, exploring the relationship between elastic coefficient and structural size and the stability of spring performance [3].

With the help of Professor Jia Yunfei's research group at East China University of Science and Technology, in-situ mechanical properties testing of micro-nano springs was conducted. It was these early data that supported the idea of making high-performance sensors.

In the early stage, the process of two-photon polymerization processing of the fiber end face was improved and a special fixture adapted to the fiber was designed, which greatly simplified the experimental process and improved the success rate of processing. Based on this processing technology, they have successively developed optical fiber refractive index sensors [4] and optical fiber temperature sensors [5], accumulating relevant technical experience for this new research.

According to reports, the team will continue to focus on micro-nano instruments and micro-nano technology, explore more deeply the design and preparation of high-performance sensors, and further expand the application scenarios of fiber-optic spring sensors, including designing new sensing units with better mechanical properties and developing More advanced 3D structure processing technology, exploring better optical sensing principles, etc.

"High-performance sensors are an important tool for basic and applied research. We look forward to applying them to front-line scientific research in the future to discover more interesting physical phenomena such as nonlinear airflow forces," Qiu Min finally said.