Droplet Position Research Articles

Time-resolved 3D visualization of liquid jet breakup and impingement behavior in a shallow liquid pool

A new three-dimensional laser-induced fluorescent (3D-LIF) technology to obtain the hydrodynamic behavior of liquid jets in a shallow pool were developed. In this technology, firstly, a refractive index matching was applied to acquire a clear cross-sectional image. Secondly, a series of cross-sectional images was obtained by using a high-speed galvanometer scanner. Finally, to evaluate the unsteady 3D interface shape of liquid jet, a method was developed to reconstruct 3D shapes from the series of cross-sectional images obtained using the 3D-LIF method. The spatial and temporal resolutions of measurement were 0.21 × 0.21 × 1.0 mm/pixel and 2 ms, respectively. The shape of a 3D liquid jet in a liquid pool and its impingement, spreading and atomization behavior were reconstructed using the proposed method, successfully. The behaviors of atomized particles detached from the jet were obtained by applying data processing techniques. Diameters distribution and position of atomized droplets after detachment were estimated from the results.

Ternary modeling of the interaction between immiscible droplets in a confined shear flow

The interaction between two immiscible droplets in a confined shear flow is studied numerically by using a ternary diffuse-interface model. Three different types of interaction are observed: (1) reverse motion, (2) pass-over motion, and (3) rotary Janus motion after wetting. We investigate the effects of initial droplet position, capillary number, and confinement on the droplet trajectory and the deformation.

3D printed mounts for microdroplet resonators.

Liquid microdroplet resonators provide an excellent tool for optical studies due to their innate smoothness and high quality factors, but precise control over their geometries can be difficult. In contrast, three dimensional (3D) printed components are highly customizable but suffer from roughness and pixelation. We present 3D printed structures which leverage the versatility of 3D printing with the smoothness of microdroplets. Our devices enable the reliable creation of microdroplet resonators of varying shapes and sizes in an ambient environment, and our coupling scheme allows for high control over droplet position.

A tripodal wheeled mobile robot driven by a liquid metal motor.

As a novel driving concept, the liquid metal motor (LMM) has been regarded as a promising actuator due to its unique traits, such as infinitely variable speed, lack of transmission chain, convenient maintenance, and silence. However, at present, driving devices based on this material are still in the preliminary and rudimentary stage, and representative application examples are scarce. Therefore, an 8-shaped tripodal wheeled mobile robot (WMR) completely driven by a LMM is designed in this study to further prove the practicability of this material. Through combining the Marangoni surface flow on a liquid metal droplet (LMD) caused by an electrochemical reaction and the eccentric torque generated by the change in droplet shape and position, the two independently driven wheels of the mobile robot are actuated at differential moving speeds. Additionally, a matching control module, a cell phone application, and a battery have been developed and added for wireless control of three types of driving functions (moving forward, steering, and stopping). It is expected that this work could further advance the development and application of LMMs and bring new ideas to the design of WMRs.

Acousto-Pi: An Opto-Acoustofluidic System Using Surface Acoustic Waves Controlled With Open-Source Electronics for Integrated In-Field Diagnostics.

Surface acoustic wave (SAW) devices are increasingly applied in life sciences, biology, and point-of-care applications due to their combined acoustofluidic sensing and actuating properties. Despite the advances in this field, there remain significant gaps in interfacing hardware and control strategies to facilitate system integration with high performance and low cost. In this work, we present a versatile and digitally controlled acoustofluidic platform by demonstrating key functions for biological assays such as droplet transportation and mixing using a closed-loop feedback control with image recognition. Moreover, we integrate optical detection by demonstrating in situ fluorescence sensing capabilities with a standard camera and digital filters, bypassing the need for expensive and complex optical setups. The Acousto-Pi setup is based on open-source Raspberry Pi hardware and 3-D printed housing, and the SAW devices are fabricated with piezoelectric thin films on a metallic substrate. The platform enables the control of droplet position and speed for sample processing (mixing and dilution of samples), as well as the control of temperature based on acousto-heating, offering embedded processing capability. It can be operated remotely while recording the measurements in cloud databases toward integrated in-field diagnostic applications such as disease outbreak control, mass healthcare screening, and food safety.

The dynamics of bouncing, partially coalescing, liquid metal droplets in a viscous medium

Planar partial coalescence is a phenomenon in which a droplet at a free surface or interface between two fluids coalesces into the plane surface producing a smaller droplet rather than coalescing completely. This smaller, ‘daughter’ droplet will be driven towards the interface by gravity and capillary forces resulting in a cascade effect of progressively small daughter droplets until the Ohnesorge Number approaches $\sim$ 1 and the cascade terminates with a full coalescence event. This paper utilizes a room temperature liquid metal alloy composed of gallium, indium and tin to study partial coalescence in a viscous quiescent medium and observed bouncing of the coalescing droplets on the interface. We observed the event using high speed videography measuring effects such as the droplet to daughter droplet ratio, droplet velocities, droplet bounce heights and coefficients of restitution for the bouncing event. An existing model (Honey & Kavehpour, Phys. Rev. E, vol. 73, 2006) from our group was used, validated and expanded upon to include buoyancy effects to estimate the initial velocity of the droplet and we developed two new models for the droplet travel and maximum bounce height. The first utilizes the Stokes model for drag to moderate success while the second utilizes a model from Beard & Pruppacher (J. Atmos. Sci., vol. 26, 1969, pp. 1066–1072) and a fourth-order Runge–Kutta numerical integration scheme to predict the droplet velocity and position as functions of time. Additionally the coefficient of restitution was determined from the model using a shooting method technique in tandem with measured data to find a coefficient of restitution value of $A = 0.27 \pm 0.06$ . This ‘bouncing drop’ phenomenon continues in a quiescent viscous fluid to the sub-micron scale and was facilitated by the material properties of the liquid metal including the high density, moderate viscosity and particularly high interfacial tension.

On the Optimal Modes for Glucose Droplet Sensing Based on Multi-Modes

In this paper, we demonstrate the optimal multi-modes for glucose droplet (GD) sensing depending on sample (glucose droplet) positions in a half-wavelength ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> /2) microstrip resonator. Using two different GD positions, i.e., an edge and a center position, in the resonator, the resonant behavior of microwave multi-modes for GD sensing was investigated. GDs with three different concentrations, i.e., 0.1, 0.2, and 0.3 g/ml, were tested in the present experiment. With increasing GD concentration, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{21}$ </tex-math></inline-formula> -level of the edge-positional GD gradually was declined at all resonant modes. For a center-positional GD, the frequency not only was deviated into the high-frequency region, but its <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}_{21}$ </tex-math></inline-formula> -level also gradually was declined for the second and fourth modes. Based on the experimental results, the resonant behavior indicated the edge-(center-) positional GD had the resolution (~0.01/gmL <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> or ~0.05 dB/gmL <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">−1</sup> ) at the second (second) and third (fourth) modes, respectively. Furthermore, we showed that the edge-positional GD sensing is more suitable when the resonator is associated with an active circuit system, and simultaneous sensing is also possible for different positions in multi-modes.

Millimeter-wave electromagnetic monitoring for liquid metal droplet-on-demand printing

As new printing approaches emerge, in situ diagnostics to monitor the print quality in real-time become essential for long-term monitoring and feedback control. In this article, we present a millimeter-wave electromagnetic monitoring approach for liquid metal droplet-on-demand printing to support the high-speed and real-time evaluation of droplet ejection. An open-ended rectangular waveguide is placed perpendicular to a jetted droplet stream and operated at a continuous-wave frequency of 40 GHz. Liquid metal droplets with diameters as low as 1.2 mm are characterized, and droplet jetting events on the order of 500 μm are detected at ejection rates up to 80 Hz. The measured results demonstrate that trends at the macro-level (large-scale print variation and anomalies at the nozzle tip) as well as micro-level (droplet size, position, and dynamics) can be detected using this technique.

A Sessile Drop Method for Facile and Robust Spheroid Cultures

AbstractIn this work, an innovative sessile drop (SD) strategy for facile, robust, and long‐term cell spheroid cultures is proposed. Unlike the conventional hanging drop (HD) method, this strategy employs a superhydrophobic perforated microwell plate (SHPMP) to spatially confine arrays of cell suspension droplets for 3D spheroid cultures. Benefitting from a combination of upward positioning of cell suspension droplets and non‐wettability of the microwell surface, the SD culture platform not only maintains the technical simplicity and high oxygen availability of the HD method, but also provides high mechanical stability and ease of medium exchange, thus enabling a robust long‐term spheroid culture in a high‐throughput manner. In a proof of concept application, the convenience and simplicity of the SD approach is demonstrated by producing 384 uniform spheroids of MHCC97H cell with SHPMP. To validate the utility of this method, it is further demonstrated that spheroids fabricated by this method can be used for drug testing. With convenient liquid handling, low reagent/sample consumption, high droplet stability, adequate oxygen supply, and good in situ monitoring compatibility, the SD platform is beneficial for long‐term cell spheroid culture in a high throughput manner and provides a powerful tool for tissue engineering, regenerative medicine, and drug screening.

In‐depth investigation of incremental layer build‐up from dried deposited droplets

AbstractThis study presents an approach to closely investigate the process of incremental layer build‐up from convectively dried sessile droplets. In a droplet‐by‐droplet manner, three layers of sodium benzoate solution droplets are consecutively applied on a glass substrate and successively dried in a laboratory‐scale drying chamber. The morphology of the simplified layer structure that incrementally forms from the individual deposits is analyzed via white‐light interferometry, after every addition of a new droplet. Furthermore, complementary X‐ray micro‐tomography measurements of the deposits were carried out to monitor inner structure development. Recurring steps and phenomena throughout the experimental build‐up process were identified and could be characterized through these methods. Moreover, these phenomena could be linked to specific combinations of droplet positioning and drying conditions. Finally, the overall influence of convective drying parameters as well as droplet properties on the layer formation process was investigated with regard to morphology and porosity.

Patterned superhydrophobic/superhydrophilic SERS sensors fabricated by femtosecond laser for precise positioning and ultra-sensitive detection

Superhydrophobic surface-enhanced Raman scattering (S-SERS) sensors enhance detection performance by concentrating analyte molecules into small areas, which have been widely used in biochemical analysis. However, the uncertainty of droplet position on the superhydrophobic surface limits the application. In this paper, patterned superhydrophobic/superhydrophilic SERS (PS-SERS) sensor is fabricated by femtosecond laser, and the R6G solution is distributed homogeneously in the superhydrophilic zone after evaporation enrichment. On the PS-SERS sensor with the diameter of 400 μm circular superhydrophilic pattern, the detection sensitivity of R6G molecules is as low as 10−13 M. The results provide an efficient way for ultratrace molecular detection.

Characterization and verification of astigmatic interferometric particle imaging for volumetric droplet 3D position and size measurement

Sprays generated by atomization processes have been used in a wide range of fields. The size and spatial distribution of volumetric droplets in a spray are critical parameters in industrial applications. Astigmatic interferometric particle imaging (AIPI), an extension of the traditional interferometric particle imaging (IPI), is developed to simultaneously measure the size and 3D position of volumetric droplets in a sparse spray. Based on the generalized Huygens–Fresnel integral and the transfer matrix, the size and depth position of droplet can be respectively extracted from the fringe spacing and orientation of interferogram in AIPI. An AIPI setup is established to characterize droplets in a sparse spray generated by a nozzle with the AIPI calibration procedure adopted. The measured parameters by AIPI are compared with those obtained synchronously by digital inline holography, which is regarded as a standard measurement technique. Results show that the average deviation values of droplet size and depth position are respectively 3.8% and 6.8%. AIPI has been demonstrated with high accuracy in simultaneous 3D positions and size measurements.

Effect of different excitation currents on droplet transfer and weld formation of Al-Mg-Si alloy CMT MIX + Synchropulse welded joint

In this paper, the droplet transfer behavior, molten pool flow and weld formation of Al-Mg-Si alloy CMT MIX + Synchropulse welded joint under different excitation currents were studied. The external magnetic field (EMF) can effectively suppress the porosity and macroscopic pore (＞200 μm), especially at 20 A and 30 A excitation currents level. The EMF significantly reduces the droplet size (0 A → 20 A, 1.6 mm → 1.4 mm), which reduces the droplet heat and the impact force on the molten pool. The EMF changes the impact position of the molten droplet on the molten pool, making the temperature distribution on both sides of the molten pool inconsistent, which finally leads to asymmetrical weld cross-sectional morphology.

Machine vision-based driving and feedback scheme for digital microfluidics system

Abstract A digital microfluidic system based on electrowetting-on-dielectric is a new technology for controlling microliter-sized droplets on a plane. By applying a voltage signal to an electrode, the droplets can be controlled to move, merge, and split. Due to device design, fabrication, and runtime uncertainties, feedback control schemes are necessary to ensure the reliability and accuracy of a digital microfluidic system for practical application. The premise of feedback is to obtain accurate droplet position information. Therefore, there is a strong need to develop a digital microfluidics system integrated with driving, position, and feedback functions for different areas of study. In this article, we propose a driving and feedback scheme based on machine vision for the digital microfluidics system. A series of experiments including droplet motion, merging, status detection, and self-adaption are performed to evaluate the feasibility and the reliability of the proposed scheme. The experimental results show that the proposed scheme can accurately locate multiple droplets and improve the success rate of different applications. Furthermore, the proposed scheme provides an experimental platform for scientists who focused on the digital microfluidics system.

DC Breakdown Characteristic of Air Gap With Water Droplets

Under rain conditions, the sliding water droplets on the insulator’s surface shorten the air insulation distance between the sheds and even cause a flashover. In order to study the mechanism of air gap breakdown with rain droplets, the breakdown process was recorded by a high-speed camera and the influences of rain water droplets (conductivity, volume, and position) were studied. The experimental results show that the impact of the water droplets on the direct current (dc) breakdown voltage is related to the air gap length. The shorter the air gap, the greater the drop of the flashover voltage. Although water conductivity does not affect the breakdown voltages, the arc development path is related to it. This research has significance for revealing the rain flashover mechanism of insulators.

Silicone oil obscuring vision by migrating behind an intraocular lens in an intact capsular bag after previous retinal detachment surgery

Silicone oil is used on a regular basis during vitrectomy for the surgical management of complex or recurrent retinal detachment. Incomplete removal of particles can cause postoperative complications. A 68-year-old pseudophakic patient with previous retinal detachment surgery presented with reduced vision. On examination, silicone oil could be visualized lodged between the hydrophobic intraocular lens (IOL) and intact posterior capsule. YAG laser capsulotomy was performed to clear the visual axis and disperse the oil droplets into the vitreous cavity. Corrected distance visual acuity improved to 6/6 and symptoms resolved. Silicone oil can accumulate in the capsular bag after vitrectomy even in patients with an intact posterior capsule. It is important to ascertain the type of IOL material used during cataract surgery and to properly identify the positioning of the oil droplets to safely treat the patient and avoid unnecessary complications.

Astigmatic dual-beam interferometric particle imaging for metal droplet 3D position and size measurement.

We propose astigmatic dual-beam interferometric particle imaging (ADIPI) to simultaneously measure the three-dimensional (3D) position and size of spherical metal droplets. A theoretical model reveals that the orientation and spacing of the ADIPI fringes generated from the two reflections propagating through an astigmatic imaging system relate to the depth position and size, respectively. Proof-of-concept experiments on micron-sized gallium droplets are performed, and the tilted fringes in elliptical patterns are observed in the ADIPI interferogram, confirming theoretical predictions. Droplet 3D position and size are determined with ADIPI, and the relative discrepancies are within 5% and 2% compared to those with a dual-view digital inline holography system, demonstrating the feasibility and high accuracy of ADIPI.

Precise Droplet Manipulation Based on Surface Heterogeneity

ConspectusThe control of droplet behavior has been investigated for several centuries due to its universality and significance in nature, daily life, and industrial applications. Diverse strategies have been developed and employed to regulate droplet behavior, and great progress has been achieved such as accelerated droplet bouncing, controlled droplet dispensing, and enhanced droplet adhesion. Compared with the strategies utilizing external fields including magnets, heat, light, and electricity as well as various topological structures, the heterogeneous wettability surface provides a facile and efficient way to manipulate droplet behavior and thus has aroused increasing research interest in many fields.The heterogeneous wettability substrate integrates wettable and less wettable regions onto an individual surface, with rational design and a geometrical arrangement. Therefore, it shows more precise, diverse, and controllable interactions with droplets than homogeneous surfaces. In addition to confining the droplet position and morphology and regulating the droplet dynamics, the nonuniformity in wettability can induce a difference in electrical conductivity, nucleation energy barriers, and so forth, which greatly extends its application in the fields of functional materials deposition and device fabrication. In this Account, recent progress in the precise droplet manipulation based on heterogeneous wettability is summarized. First, the principle of droplet control by a heterogeneous surface and the development of the green printing technique based on this principle are introduced. Then the droplet manipulation, categorized as droplet patterning, droplet isolation, and droplet dynamics, is described. Meanwhile, related applications, such as high-capacity information coding, highly sensitive detection, and high-performance optic/electric device fabrication, are demonstrated. In addition, the general principle for controlling the droplet impact using heterogeneous wettability substrates is highlighted. Finally, a brief summary of this Account, with challenges and opportunities in this field, is proposed.

Self-assembly of superstructures at all scales

Summary Controllable assembly of molecular and nanoscale building blocks into uniform superstructures up to bulk dimensions remains a key challenge in the next phase of nanotechnology development. Here, we report the self-assembly of superstructures at all scales by taking advantage of the partial miscibility of water and 1-butanol to generate transient aqueous emulsion droplets that can encapsulate the target materials and introduce them into template holes. Further diffusion of water into 1-butanol depletes the emulsion droplets, assembling the building blocks into one well-defined superstructure in each hole. Superstructuring of various types and shapes of nanoparticles, biomolecules, and inorganic compounds could be achieved without the need for surfactants and chemical modifications. The versatility, scalability, low-cost, and accurate positioning are crucial advantages for the future development of advanced precision manufacturing.

Dynamic analysis of the ultrasonic-frequency pulsed GMAW metal transfer process

Current waveform control method was of great significance to welding quality of GMAW process. In order to study the influence of a novel control method of ultrasonic-frequency-pulse on metal transfer process, a high-speed-camera was employed for droplet monitoring, and two image processing algorithms based on MATLAB were proposed to extract the droplet size and position. Dynamic analysis based on pulsed GMAW in the mode of ‘one drop per pulse’ was carried out. The results showed that compared with the conventional pulsed GMAW, the ultrasonic-frequency-pulse promoted droplet falling speed without changing droplet size under the same stable condition, the droplet detaching mean velocity increased by 13 % at the maximum before the detachment, and the droplet driving force in peak time duration increased by 19.9 % at most after the detachment. The study helped to precisely control the pulsed GMAW metal transfer process.