Measurement Methods for Droplet Adhesion Characteristics and Micrometer-Scale Quantification of Contact Angle on Superhydrophobic Surfaces: Challenges and Opportunities.

Inspired by nature, superhydrophobic surfaces have been widely studied. Usually the wettability of a superhydrophobic surface is quantified by the macroscopic contact angle. However, this method has various limitations, especially for precision micro devices with superhydrophobic surfaces, such as biomimetic artificial compound eyes and biomimetic water strider robots. These precision micro devices with superhydrophobic surfaces proposed a higher demand for the quantification of contact angles, requiring contact angle quantification technology to have micrometer-scale measurement capabilities. In this review, it is proposed to achieve micrometer-scale quantification of superhydrophobic surface contact angles through droplet adhesion characteristics (adhesion force and contact radius). Existing contact angle quantification techniques and droplet characteristics' measurement methods were described in detail. The advancement of micrometer-scale quantification technology for the contact angle of superhydrophobic surfaces will enhance our understanding of superhydrophobic surfaces.

A multi-task model for reliable classification of thyroid nodules in ultrasound images.

Thyroid nodules are common, and patients with potential malignant lesions are usually diagnosed using ultrasound imaging to determine further treatment options. This study aims to propose a computer-aided diagnosis method for benign and malignant classification of thyroid nodules in ultrasound images. We propose a novel multi-task framework that combines the advantages of dense connectivity, Squeeze-and-Excitation (SE) connectivity, and Atrous Spatial Pyramid Pooling (ASPP) layer to enhance feature extraction. The Dense connectivity is used to optimize feature reuse, the SE connectivity to optimize feature weights, the ASPP layer to fuse feature information, and a multi-task learning framework to adjust the attention of the network. We evaluate our model using a 10-fold cross-validation approach based on our established Thyroid dataset. We assess the performance of our method using six average metrics: accuracy, sensitivity, specificity, positive predictive value, negative predictive value, and AUC, which are 93.49, 95.54, 91.52, 91.63, 95.47, and 96.84%, respectively. Our proposed method outperforms other classification networks in all metrics, achieving optimal performance. We propose a multi-task model, DSMA-Net, for distinguishing thyroid nodules in ultrasound images. This method can further enhance the diagnostic ability of doctors for suspected cancer patients and holds promise for clinical applications.

Exploration of Sweeping Effect: Droplet Coalescence Jumping of a Rolling and Static Droplet.

The sweeping effect of merged droplets plays a key role in enhancing application performance due to the continuing coalescence caused by the horizontal jumping velocity. Most studies focused on static droplet coalescence jumping, while moving droplet coalescence is poorly understood. In this work, we experimentally and numerically study the coalescence of a rolling droplet and a static one. When the droplet radius ratio is larger than 0.8, as the dimensionless initial velocity increases and the vertical jumping velocity first decreases and then increases. The critical dimensionless initial velocity Vc* corresponding to the minimum vertical jumping velocity could be estimated as 0.9(rs2rm2). When the droplet radius ratio is smaller than 0.8, the dimensionless initial velocity has a positive effect on the vertical jumping velocity. The mechanism of the vertical jumping velocity can be attributed to two parts: liquid bridge impact and retraction of the merged droplet. The squeezing effect generated by the initial velocity between the two droplets promotes the growth of the liquid bridge and enhances the impact effect of the liquid bridge but weakens the upward velocity accumulation caused by the retraction of the merged droplets. However, different from the vertical jumping velocity, the horizontal jumping velocity is approximately proportional to the dimensionless initial velocity. The outcome of our work elucidates a fundamental understanding of a rolling droplet coalescing with a static one.

Exploring the Effects of Liquid Marbles' Deformation on Their Rolling Resistance.

Liquid marbles (LMs) are nonwetting droplets manufactured by encapsulating droplets with micro- or nanoscale particles. These marbles are widely used as transport carriers for digital microfluidics due to their rapid displacement velocity and leak-free transport. An improved understanding of the resistance mechanism of rolling LMs is crucial for their transport and manipulation. In this study, we investigated the rolling resistance of LMs obtained with different powders and volumes using a high-speed camera. Our findings suggest that the deformation of liquid marbles would hinder their rolling by a resistance torque. To depict this resistance effect, we propose a theoretical model (f∼λ(ε-12Bo1/2ε2+14Boε3)), where f is the rolling resistance of marbles, λ is the deflection coefficient, Bo is the Bond number, and (ε is the contact surface deformation) that accurately predicts the relationship between deformation and rolling resistance, which is supported by our experimental results. To further validate our theoretical model, we conducted three independent experiments: shape detection of prepared LMs, measuring the elastic force of LMs, and detecting the diffusive motion of the encapsulating particles. Furthermore, we discuss three factors that affect the rolling resistance: the volume of the marbles, the type and size of the encapsulating particles, and the substrate roughness. This comprehensive study not only generalizes the mechanism of deformation hindering the rolling of liquid marbles but also provides a theoretical framework to predict the relationship between the deformation and rolling resistance. These findings have practical implications for improving the manipulation efficiency and advancing the use of LMs as microfluidic carriers.

Vision-Based Contact Adhesion Measurement Method on Soft Matter Surfaces.

Adhesion property measurements contribute to a comprehensive understanding of the mechanical properties of soft matters. Indentation tests are a common method for measuring the adhesion force. However, indenters generally have a large volume and a small sensing angle and, thus, are not conducive to local detection in high-precision environments. Here, we propose a vision-based contact adhesion measurement (VisCAM) method to achieve the contact image and adhesion force on soft matter surfaces from the perspective of indentation direction. The coupling of the 7.6 mm diameter probe and a flexible fiber makes the system similar to a miniaturized endoscope. Classical contact theories and finite element models are used for the contact mechanics analysis of silicone rubber. The image grayscale-load mathematical model is constructed based on the change in contact light spot. Finally, the uncertainty of the system is less than 4%, and the measurement error is 0.04 N. In-vitro kidney indentation experiments showed that the local adhesion force measurement of soft tissues can be completed. Our method provides better solutions for understanding the adhesion properties of soft matters.

Measuring the Adhesion Force and the Spreading Radius between Droplets and a Solid Surface during Short-Time Spreading.

When a droplet contacts a solid surface, the liquid spreads over the solid surface to minimize the total surface energy. This phenomenon is widespread in industrial production and nature, so research on droplet spreading is of great significance. Here, the adhesion force and the spreading radius during droplet spreading can be quantified using a highly sensitive photoelectric method. It is possible to study droplet spreading from two dimensions at the microscale. The adhesion force is measured by an optical lever, and the spreading radius is measured by an ultrafast electrical method. The measurement method allows the force resolution and the space-time resolution to reach the nanonewton lever and the nanosecond lever, respectively. We obtain the maximum spreading radius and the maximum adhesion force during short-time spreading through our technique. Moreover, we numerically simulate the droplet spreading process through the lattice Boltzmann solver and confirm the observed results. This study provides a new experimental technique for studying droplet spreading dynamics from multiple perspectives, which can deepen our understanding of droplet spreading and provide guidance for the development of new techniques.

A disturbance suppression micro-Newton force sensor based on shadow method.

The precision of micro-force measurement depends on the force sensor sensitivity and the environmental disturbance magnitude. However, micro-force sensors generally have the poor anti-disturbance ability. Inspired by the shadow formed by water striders walking on water surface under sunlight, a viscoelastic-polymer micro-force (VPMF) sensor based on the shadow method was proposed, which could suppress disturbances effectively due to the high damping ratio of 0.22. The shadow diameter change and the applied force were proportional. The experimental results indicated that the sensitivity could reach 2.15 µN/pixel with a good linear performance. Furthermore, compared with the cantilever, it was capable of the reduction of the disturbance influence by approximately 96.35%. Therefore, the VPMF sensor can be applied to reliable micro-force measurement in complex environments such as industrial sites.

Method for parallelism measurement of geometrical waveguides based on the combination of an autocollimator and a testing telescope.

Augmented reality (AR) is desperately needed in the Metaverse. The geometrical waveguide receives increased attention in AR technology as achieving high resolution, full-color display, etc. However, the stray light and ghost image problems resulting from the parallelism errors severely deteriorate the imaging quality. According to the light propagation of the waveguide, a measuring system based on the combination of the autocollimator and the testing telescope (CAT) method was proposed to measure the parallelism errors of the partially reflective mirror array (PRMA). The results indicated that this method could measure the parallelism errors precisely with the maximum repeatability of 0.63 ' ' . The method could decouple the coupling of the parallelism errors of the PRMA and the substrate surfaces to imaging quality effectively. The precise parallelism measuring is expected to contribute to mass production and low cost by promoting the waveguide design and fabrication.

Development of a nanoscale displacement sensor based on the shadow method.

A novel, to the best of our knowledge, bionic coaxial micro-displacement sensor based on the shadow method is developed and experimentally demonstrated inspired by the water strider walking on the water. The water is used as the sensitive element to measure the micro- displacement. A meniscus is formed by the superhydrophobic circular plate subjected to a coaxial displacement excitation. Then a shadow is formed because of the refraction when the parallel light illuminates the meniscus. A maximum coaxial displacement sensitivity of 62 nm/pixel over the displacement range of 50 µm is achieved experimentally. The linearity error in the measurement range was 1.58%. Therefore, it is expected that this displacement sensor can be used in many important ultraprecision measurement fields because of the advantages of the easy structure and high resolution.

High-precision and long-range measurement of micro-gear starting torque.

The ability to measure micro-starting torque is pivotal for micromechanical equipment, which has wide usage in mechanical manufacturing, electrical, electronic, and other industries. However, the measurement range of existing methods is about N⋅m or mN⋅m. There is not much research on the measurement of micro-torque starting in the µN⋅m. In this paper, a novel micro-gear starting torque measurement system, to the best of our knowledge, is proposed based on an optical lever with a long range from 1 to 10µN⋅m. The system device consists of the optical lever, cantilever, and position sensitive device. A micro-gear was used to assess the performance of the proposed method. The standard deviation of the measured starting torque is 1.2µN⋅m. The external factors that can contribute to the uncertainty of the measurement system, such as force measurement, arm of force, and repeatability, have been analyzed and quantified. The relative combined uncertainty is estimated at 3.0%, approximately.

A Micro-Displacement Measurement Based on the Shadow Technique.

The precision measurement of micro displacement is important in scientific and industrial fields. However, it is a tough challenge due to the complex design and the high cost of measuring instruments. Inspired by the shadow formed from water striders walking on a water surface under sunlight, a micro-displacement measurement method was proposed. Water strider legs with superhydrophobic properties bend the water surface. The curved surface of the water refracts sunlight, creating a shadow with a bright edge at the bottom of the pond. The shadow size is generally larger than the indentation depth of the legs from the water surface. In the micro-displacement measurement system, the applied displacement is proportional to the change in the diameter of the shadow. The presented study proposes a micro-displacement measurement procedure based on this shadow technique. The displacement sensitivity can reach 10.0 nm/pixel in the range of 5 µm. This system is simple to construct, low cost, and has high precision with good linear performance. The method provides a convenient additional option to measure micro-displacement.

Molecular Dynamics Simulation on the Wettability of Nanoscale Wrinkles: High Water Adhesion of Rose Petals.

Understanding the high water adhesion of rose petals is of great significance in artificial surface design. With all-atom molecular dynamics simulation, the wettability of nanoscale wrinkles was explored and compared to that of nanoscale strips with favorable hydrophobicity. The dewetting and wetting of gaps between nanoscale structures represent the Cassie-Baxter (CB) and Wenzel (WZ) states of the macroscopic droplet deposited on the textured surface, respectively. We uncovered the intermediate state, which is different from the CB and WZ states for wrinkles. Structures and free-energy profiles of metastable and transition states under various pressures were also investigated. Moreover, free-energy barriers for the (de)wetting transitions were quantified. On this basis, the roles of pressure and the unique structures of nanoscale wrinkles in the high water adhesion of rose petals were identified.

Liquid marbles, floating droplets: preparations, properties, operations and applications.

Liquid marbles (LMs) are non-wettable droplets formed with a coating of hydrophobic particles. They can move easily across either solid or liquid surfaces since the hydrophobic particles protect the internal liquid from contacting the substrate. In recent years, mainly due to their simple preparation, abundant materials, non-wetting/non-adhesive properties, elasticities and stabilities, LMs have been applied in many fields such as microfluidics, sensors and biological incubators. In this review, the recent advances in the preparation, physical properties and applications of liquid marbles, especially operations and floating abilities, are summarized. Moreover, the challenges to achieve uniformity, slow volatilization and stronger stability are pointed out. Various applications generated by LMs' structural characteristics are also expected.

Laplace Pressure Difference Enhances Droplet Coalescence Jumping on Superhydrophobic Structures.

Coalescence-induced droplet jumping has great prospects in many applications. Nevertheless, the applications are vastly limited by a low jumping velocity. Conventional methods to enhance the droplet coalescence jumping velocity are enabled by protruding structures with superhydrophobic surfaces. However, the jumping velocity improvement is limited by the height of protruding structures. Here, we present rationally designed limitation structures with superhydrophobic surfaces to achieve a dimensionless jumping velocity, Vj* ≈ 0.64. The mechanism of enhancing the jumping velocity is demonstrated through the study of numerical simulations and geometric parameters of limitation structures, providing guidelines for optimized structures. Experimental and numerical results indicate that the mechanism consists of the combined action of the velocity vectors' redirection and the Laplace pressure difference within deformed droplets trapped in limitation structures. On the basis of previous research on the mechanisms of protruding structures and our study, we successfully exploited those mechanisms to further improve the jumping velocity by combining the limitation structure with the protruding structure. Experimentally, we attained a dimensionless jumping velocity of Vj* ≈ 0.74 with an energy conversion efficiency of η ≈ 48%, breaking the jumping velocity limit. This work not only demonstrates a new mechanism for achieving a high jumping velocity and energy conversion efficiency but also sheds lights on the effect of limitation structures on coalescence hydrodynamics and elucidates a method to further enhance the jumping velocity based on protruding structures.

Elasticity and damping ratio measurement of droplets on super-hydrophobic surfaces.

The measurement of the droplets' elasticity is vitally important in microfluidic and ink-jet printing. It refers to the ability of the droplet to restore its original shape and strong robustness. This study investigated a novel method to measure elasticity. The plate coated with super-hydrophobic layers pressed on a droplet and the elastic force was recorded by an electronic balance. Meanwhile, a mathematical model was constructed to calculate the changes of the droplet area under the force. The measurement showed that external work mainly converts into surface energy and the damping ratio increases from 0.068 to 0.261 with the increase of mass fraction from 0 to 80 wt%. It also indicates that the novel method can accurately and efficiently measure the elasticity of droplets.

The Traceable Calibration of Thrust Stand by Electrostatic Force.

Micro thrusters have important applications in low-frequency gravitational wave detection, satellite formation, and inter-satellite laser communication, so it is necessary to accurately measure the thrust of micro thrusters with traceability. A thrust stand is a widely used micro thrust measuring device with the advantages of high resolution and large load. Traceability calibration of mechanical characteristics of thrust stand is an essential prerequisite to ensure traceability measurement of thrust. In this study, a parallel plate capacitor was used to calibrate the thrust stand by generating a micronewton electrostatic force, which could be traced to the International System of Units (SI). The constant capacitance gradient range was obtained through simulation and theoretical calculation. Moreover, the electrostatic force could be changed by standard voltage with the advantages of simple principle, instantaneous trigger, and traceability. The device could be used for traceability calibration of micro newton thrust stand due to simple assembly and short traceability path.

Enhancement and Guidance of Coalescence-Induced Jumping of Droplets on Superhydrophobic Surfaces with a U-Groove.

Coalescence-induced droplet jumping has received considerable attention owing to its potential to enhance performance in various applications. However, the energy conversion efficiency of droplet coalescence jumping is very low and the jumping direction is uncontrollable, which vastly limits the application of droplet coalescence jumping. In this work, we used superhydrophobic surfaces with a U-groove to experimentally achieve a high dimensionless jumping velocity Vj* ≈ 0.70, with an energy conversion efficiency η ≈ 43%, about a 900% increase in energy conversion efficiency compared to droplet coalescence jumping on flat superhydrophobic surfaces. Numerical simulation and experimental data indicated that a higher jumping velocity arises from the redirection of in-plane velocity vectors to out-of-plane velocity vectors, which is a joint effect resulting from the redirection of velocity vectors in the coalescence direction and the redirection of velocity vectors of the liquid bridge by limiting maximum deformation of the liquid bridge. Furthermore, the jumping direction of merged droplets could be easily controlled ranging from 17 to 90° by adjusting the opening direction of the U-groove, with a jumping velocity Vj* ≥ 0.70. When the opening direction is 60°, the jumping direction shows a deviation as low as 17° from the horizontal surface with a jumping velocity Vj* ≈ 0.73 and corresponding energy conversion efficiency η ≈ 46%. This work not only improves jumping velocity and energy conversion efficiency but also demonstrates the effect of the U-groove on coalescence dynamics and demonstrates a method to further control the droplet jumping direction for enhanced performance in applications.

Coalescence-Induced Droplet Jumping.

When two or more droplets coalesce on a superhydrophobic surface, the merged droplet can jump spontaneously from the surface without requiring any external energy. This phenomenon is defined as coalescence-induced droplet jumping and has received significant attention due to its potential applications in a variety of self-cleaning, anti-icing, antifrosting, and condensation heat-transfer enhancement uses. This article reviews the research and applications of coalescence-induced droplet jumping behavior in recent years, including the influence of droplet parameters on coalescence-induced droplet jumping, such as the droplet size, number, and initial velocity, to name a few. The main structure types and influence mechanism of the superhydrophobic substrates for coalescence-induced droplet jumping are described, and the potential application areas of coalescence-induced droplet jumping are summarized and forecasted.

Self-Enhancement of Coalescence-Induced Droplet Jumping on Superhydrophobic Surfaces with an Asymmetric V-Groove.

Coalescence-induced droplet jumping on superhydrophobic surfaces have recently received significant attention owing to their potential in a variety of applications. Previous studies demonstrated that the self-jumping process is inherently inefficient, with an energy conversion efficiency η ≤ 6% and dimensionless jumping velocity Vj* ≤ 0.23. To realize a quick removal of droplets, increasing effort has been devoted to breaking the jumping velocity limit and inducing droplets sweeping. In this work, we used superhydrophobic surfaces with an asymmetric V-groove to experimentally achieve an enhanced coalescence-induced jumping velocity Vj* ≈ 0.61, i.e., more than 700% increase in energy conversion efficiency compared with droplets jumping on flat superhydrophobic surfaces, which is the highest efficiency reported thus far. Moreover, the enhanced jumping direction shows a deviation as high as 60° from the substrate normal. The induced in-plane motion is conducive to remove a considerable number of droplets along the sweeping path and significantly increase the speed of droplet removal. Numerical simulation indicated that the jumping enhancement is a joint effect resulting from the impact of the liquid bridge on the corner of the V-groove and the suppression of droplet expansion by the sidewall of the V-groove. The transient variation of the droplet velocity and the driving force of the coalescing droplets on a surface with and without the asymmetric V-groove were revealed and discussed. Furthermore, effects of groove angle, droplet pair positions, and size mismatches on the jumping velocity and direction have been studied. The novel mechanism of simultaneously increasing the coalescence-induced droplet jumping velocity and changing the jumping direction can be further studied to enhance the efficiency of various applications.

Detection of the three-dimensional trajectory of an object based on a curved bionic compound eye.

Biological compound-eye systems have unique advantages in three-dimensional (3D) positioning based on light energy distribution. A curved compound eye was designed and manufactured to imitate a biological compound eye. To overcome the nonuniform off-axis response and enlarge the aperture of the eyelet, a novel dome light cone was designed. The dome light cone was designed as a conical structure, which consisted of a lot of fiber wires with a diameter of 6 µm. Additionally, based on the proposed biological compound-eye systems, an algorithm was proposed to obtain the 3D position of the object by analyzing the light location and intensity distribution. The effect of the illumination intensity, the position of the target's center, and the non-repeatability were evaluated. The relative standard uncertainty in the 3D position was estimated to be 8.6%. Low uncertainty verified the validity of the 3D localization algorithm.