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Climbing droplets driven by mechanical layer into transverse waves

Climbing droplets driven by mechanical layer into transverse waves

Transferring a droplet of tracer particles to a mechanical layer of the traveling wave device. Credit: Science of Science, doi: 10.1126 / sciadv.aaw0914

Modern applications use self-cleaning strategies and digital microfluids to control individual fluid droplets on flat surfaces, but existing techniques are limited by the side effects of high electrical fields and high temperatures. In a new study, Edwin De Jong and his associates in Advanced Materials, Mechanical Engineering and Complex Molecular Systems developed an innovative "mechanicalowetting" technique for controlling droplet movement on changing surfaces based on the surface tension of the interface.

To demonstrate the method, they transfer droplets using transverse waves on horizontal and vertically inclined surfaces at speeds equal to wave velocity. Scientists have occupied the fundamental mechanism of the power of the machining unit theoretically and quantitatively to determine the dependence of the phenomenon on the properties of fluid, surface energy and wave parameters. Jong et al. has shown a "mechanical layer" as a technique that can lead to a range of new applications involving droplet control through surface deflections. The survey is published today Science Sciences.

In the work, Jong et al. quantitated the dynamic push forces that led the mechanical layer by studying climbing droplets of different sizes at different tilt angles. They noticed unexpectedly large forces and were able to drive droplets even in vertical walls at considerable speeds. The droplets were able to choose contaminating particles along the road to demonstrate their capabilities in self-cleaning applications. Scientists captured the underlying mechanisms of droplet transfer numerically and theoretically to establish their dependence on multiple physical parameters. Jong et al. they expect the technique to lead a series of new applications based on three-phase contact line manipulation and surface topography switching.

Climbing droplets driven by mechanical layer into transverse waves

Transfer droplets in transverse wave surface topographies. (A) Schematic of the experimental installation of the transverse wave array. Here is the wavelength, λ is the wavelength, θ is the contact angle, d is the standard droplet size, the slope is the atmospheric pressure and Δp is the pressure difference created by a vacuum pump to transform the level PDMS a waveform structure with a wavelength dictated by the distance between the vertices of the zone. The washers in the droplet are a schematic diagram illustrating the internal flow of droplets in the center of the mass after the droplet. (B to D) Glycerol droplets containing tracer particles transferred from the wave waving device. Here, A = 4 ± 1 μm, λ = 500 μm and θY = 100 ± 2 °. In Figure S1, the tape frames overlap to create path lines, showing the flow pattern following the flow diagram according to Figure 1A. (E to G) Dynamic Dynamic Fluid (CFD) simulations of the glycerol droplet at a transverse deformed surface boundary for the same traveler waveforms (shape, wavelength, wave velocity and wavelength), droplet properties and Young experiments. The small arrows in the droplet indicate the local fluid velocity in the reference center of the mass center. Credit: Science of Science, doi: 10.1126 / sciadv.aaw0914

Scientists built a device to produce regular and controlled transverse surface waves for the experimental presentation of droplet transfer. In the mechanism of action, they reduced the pressure under a polydimethylsiloxane (PDMS) film tightened by a metal frame to create a surface waveform architecture to ensure pure transverse waves. Using the experimental device, scientists control the droplets ranging from 0.1 to 5 μL in transverse waves corresponding to 500 nm wavelength at a speed of 0.57 mm / s. equal to the speed of the applied wave. Material scientists made a combination of computational dynamic fluidity (CFD), theoretical models and single drop experiments to numerically analyze individual droplets.

During computational modeling experiments, they developed an openFOAM framework to create a simulation that best matched experiments. To understand the effectiveness of the droplet transfer device, scientists conducted a series of climbing droplet experiments and simulations with the device tilted at an angle of interest. Jong et al. showed that when the driving force for the largest droplet was greater than the gravitational force, the droplet ascended upwards, while with smaller droplets the greater gravitational force caused the droplet to slip.

Climbing droplets driven by mechanical layer into transverse waves

Transfer droplets on inclined surfaces. (A) Critical crimp angle as a function of droplet size d normalized to wavelength λ. Indicators are experimental results. error bars represent the SD of at least three measurements. The stress line corresponds to numerical results. The numerical model uses the experimental settings as an input, ie the angle Young θΥ = 68 °, the wavelength λ = 500 μm, the width Α = 4.0 ± 1.0 μm and the dynamic viscosity ν = 1 mm2 s-1 of the liquid-isopropanol). The margin of error in width is reflected by the shaded area around the main stress line (in orange). (B and C) Two droplets experiment showing droplets of size d / λ = 2,7 and 3,1 at an angle of inclination β = 13 ° [corresponding to the marked locations in (A) indicated by the dashed lines]. The arrows indicate the movement of the droplets. (D) Numerical results illustrating the change of the critical angle angle as a function of the wave velocity uwave and the wavelength A for a droplet of d / λ = 3.2 (λ = 500 μm). The marked data point corresponds to the amplitude and wave velocity of the experiments presented in (A). Credit: Science of Science, doi: 10.1126 / sciadv.aaw0914

During the experiments, scientists identified a "restorative force" that drifted the droplet movement and quantified this by modeling the droplet as a spherical cap. They showed the force of dynamic tightening that balanced the neutralizing forces, which involved static attachment, gravity and viscous forces during the transfer of droplets.

They received the highest forces that could be created in the device for contact angles close to 65.5 degrees. In addition, droplets in waves could overcome significant gravitational forces even to climb vertical surfaces at a speed of 0.57 mm / s. Jong et al. showed droplets of millimeter size that could be transferred upside down. to show phenomena that so far lacked experimental demonstration.

Climbing droplets driven by mechanical layer into transverse waves

Numerical and theoretical analysis of climbing droplets. The top row shows simulation snapshots (cross sections and peak views) and the lower row shows theoretical results from the theory of triphasic line sections of a droplet of 0.15 μl (d / λ = 2.1) (A and B) of 0.35 μl droplets (d / λ = 2,7) (C and D) for wavelength A = 5 μm. The states in (A) and (C) correspond to zero velocities and slope, uwave = 0 mm s-1 and β = 0 and the states in (B) and (D) s-1 β ≈ βcrit ≈ 48 ° and 7 °, respectively. The height of the surface ridges (upper row) is indicated by a gray scale in the upper face and is excessive in the cross-sectional view. Credit: Science of Science, doi: 10.1126 / sciadv.aaw0914

During in vitro experiments (in a laboratory), the scientists formed the wave travel device using a conveyor belt manufactured by machining with an integrated velocity control located in a vacuum chamber. They deposited the PDMS film made with a pivot coating into an aluminum frame positioned at the top of the exposed portion of this belt. The low pressure created on the device allowed the PDMS film to be pressed into the zone and the scientists checked the wavelength by controlling the pressure level within the chamber.

They tried the device using various liquids including water, isopropanol and mineral oil to show the method as a powerful, consistent and reproducible process for droplet movement in all cases. Jong et al. confirmed this effectiveness by spraying droplets of various sizes at the same time in the moving wave. The observed flexibility of the engineered layer was remarkable compared to previous methods with specific requirements. When looking at the self-cleaning properties of the engineered road surface, the researchers found the ability of the droplets to clear the surface cleanly from contamination. The technique allowed controlled droplet movement to collect residues at defined locations, as opposed to previous self-cleaning procedures based on rigid and static hydrophobic surfaces.

Climbing droplets driven by mechanical layer into transverse waves

Transfering shellfish droplets to the surface of the mechanical layer of the waveguide device. Credit: Science of Science, doi: 10.1126 / sciadv.aaw0914

In this way, Jong et al. demonstrated experimental droplet-suction motion on mechanical layer surfaces and underlined the required topographical deformation in the surface three-phase line to influence the local surface tension balance and achieve motion. The present arrangement is limited as a test-of-concept experimental device to the mechanical layer mechanism. Scientists are seeking to optimize the system and build devices that have topographies that can be mechanically deformed in response to external stimuli such as light, magnetic fields and temperature. They can also control splitting and merging droplets by creating surfaces with two traveling waves traveling to or away from each other.

Edwin Jong and his colleagues believe that the mechanical layer can be fully explored to open up new opportunities for handling high precision droplets in a variety of medical and industrial applications based on the method detailed in the study. The droplets driven by the mechanical layer will find future applications in microarray to diagnose and manipulate / analyze cells and as self-cleaning devices in medicine, marine sensors, windows and solar collectors while finding applications in the cool harvest.

Using waves to move droplets

More information:
Edwin De Jong et al. Climbing droplets driven by a mechanical layer in transverse waves, Science Sciences (2019). DOI: 10.1126 / sciadv.aaw0914

Helen Song et al. Reactions to droplets in microcurrent channels, Angewandte Chemie International Edition (2006). DOI: 10.1002 / anie.200601554

Ali Hashmi et al. Leidenfrost Rise: Beyond the droplets, Scientific reports (2012). DOI: 10.1038 / srep00797

M. K. Chaudhury et al. How To Make Water Run Uphill, Science (2006). DOI: 10.1126 / science.256.5063.1539

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