Liquid climbers: manipulating water drops to create labs on a chip and self-cleaning technology

    “Once it started to rain and did not stop for four months. During this time, we learned all kinds of rain: direct rain, slanting rain, horizontal rain, and even rain that goes from the bottom up ”(Forrest Gump, 1994).

    Of course, we remember that Forrest had a special look at the world around him. Speaking about the rain "from the bottom up," he meant raindrops bouncing off the surface of the reservoir. After all, water cannot just move up, right? First, maybe. Secondly, upward is not the only direction of water movement. Thirdly, the direction can be controlled. Manipulations with tiny drops of water will allow you to create laboratories on a chip and give these or those materials self-cleaning properties. Previously, these statements were only a theory, but recently they have been confirmed in practice, which we will talk about today. What is a laboratory on a chip, how can things clean themselves, and how did scientists manage to tame drops of water? The answers to these questions are hidden in the report of scientists. Go.

    Study basis

    We increasingly hear the word "control." Scientists are trying to control almost everything that can help achieve successful results of a particular study: control of spins, control of molecules, control of the auditory cortex of the mouse brain, etc. In the case of self-cleaning technologies and digital microfluidics, control of the droplets of the liquids used in them is required.

    At the moment, there are already a number of technologies for manipulating drops, however, all of them have quite serious drawbacks: complex electrode patterns, too high temperature ( Leidenfrost installation * ), low molecular displacement velocity, the need for piezoelectric substrates, etc.
    The Leidenfrost * effect is a phenomenon when, upon contact of a liquid with a very hot body (temperature above the boiling point of a liquid), an insulating vapor layer is formed to prevent the liquid from boiling off quickly.
    However, in the study we are examining today, scientists were able to overcome all of the above shortcomings by applying a new method - " mechanowetting ". This technique allowed the drop to be moved along horizontal and inclined surfaces by means of transverse surface waves. In this case, the droplet velocity is equal to the wave velocity. More details about the results of observations below.

    Research results

    To demonstrate the unusual droplet route, scientists have developed a device that generates conventional and controlled transverse surface waves ( 1a ).

    Image No. 1 The

    wave-like structure of the surface, which is clearly visible on 1a, was obtained by reducing the pressure under the PDMS * film with a thickness of 50 μm. Because of this, the film is pressed against a moving belt with transverse protrusions.
    PDMS * - polydimethylsiloxane, (C 2 H 6 OSi) n .
    The wavelength of the film is determined by the distance between the protrusions on the belt, and the wave amplitude (3 to 5 μm) is controlled by vacuum pressure. Scientists have found that individual drops with a volume of 0.1 ... 5 μl (microliter) at transverse waves with a wavelength of 500 μm move at a speed of 0.57 mm / s, which is equal to the speed of the wave used.

    Movement of a droplet with a volume of 1.3 μl (corresponds to images 1d - 1d ).

    Next, scientists conducted CFD modeling (computational fluid dynamics) in conjunction with practical experiments and theoretical foundations in order to better understand the nature of the driving force, quantify it and, naturally, better control the entire system.

    The numerical description of a single drop was performed using a CFD model with vertically deforming boundaries (OpenFOAM framework).

    CFD model of a droplet with a volume of 1.4 μl (corresponds to images 1e - 1g ).

    Comparing the first and second videos, as well as pictures of real drops and simulated images, we see that the results are perfectly matched. Modeling fully confirms practical observations.

    Scientists analyzed the particles inside the droplet by comparing the observations with the internal droplet velocity in the CFD model, as a result of which rotational motion was detected.

    Image No. 2

    Further, the scientists complicated the task for the drops by changing the slope of the surface ( 2a ) so that the gravitational force does not become more than the driving force of the drops. On chart 2athe results of a series of experiments are shown in which a critical value of the angle of inclination corresponds to each test drop volume. A certain tendency is being observed: the value of the critical angle of inclination decreases with increasing droplet volume. This is explained quite simply: the force of gravity increases much faster than the driving force of a drop, which acts through a three-phase contact line (liquid - liquid - air).

    It was also found that the value of the critical angle does not decrease linearly (uniformly), instead, there is a sharp decline, visible on graph 2a .

    Two drops of different volume on a surface with an inclination angle of 13 ° (corresponds to images 2b and 2c ).

    Since the driving force of the larger drop (3.1) is greater than the gravitational one, the drop moves up. And the driving force of the smaller drop (2.7) is less than the gravitational one, therefore the drop rolls down.

    It should be noted that the input data for the model corresponded to the parameters of the real setup and the droplets used in practical experiments.

    Next, scientists checked how speed and amplitude affect the value of the critical angle of inclination. As seen from the diagram on 2d, the critical angle decreases with increasing wave velocity. It was also found that at a sufficiently high wave speed and at a lower amplitude range, the driving force becomes insufficient for droplet transfer. This is due to the increased viscous resistance associated with an increase in viscous dissipation inside the droplet.

    The correlation between dissipation and droplet velocity is due to the fact that an increase in the droplet transfer rate necessarily leads to an increase in the flow rate inside the droplet due to the nature of the droplet motion (rotational). In addition, an increase in the wave amplitude leads to a linear increase in the critical angle.

    The next step in the study was to clarify the reasons for the oscillatory nature of the critical angle of inclination and its possible connection with the shape of the droplets and the contact lines during their movement.

    Image No. 3

    Scientists decided to consider two options for drops: 2.1, for which the critical angle reaches a maximum (upper row at 3a and 3b ), and 2.7, for which the critical angle reaches a minimum (upper row at 3d and 3c ).

    At a zero angle of inclination of the droplet ( 3a and 3c) have a symmetrical shape. When the wave begins to move, the device tilts, which leads to a distortion of the position of the drop relative to the ridges. Because of this, the droplet shape becomes asymmetric ( 3b and 3d ).

    Changing the shape of the droplet takes the system out of balance and activates the elastic force * , which controls the movement of the droplet.
    Elastic force * - during the deformation of the body, an elastic force arises, which tends to return to its previous shape (i.e., to its original state).
    Quantitative determination of the elastic force was carried out by means of theoretical modeling based on an instantaneous change in the local contact angle upon distortion of the droplet shape. In the model medium, droplets were described as spherical particles, and the magnitude of the distortion of the droplet shape when the center of mass of the same droplet was shifted relative to the initial (equilibrium) state was fixed. Given this, scientists calculated the force per unit length (tension) arising from the imbalance of surface tension forces (blue arrows in image No. 3) on the contact line.

    The total force was obtained by integrating the stresses along the three-phase line, resulting in a net force (red arrow in image No. 3).

    The theoretical model confirmed that in the initial (equilibrium) state, the tensions of the three-phase lines are symmetrical. And during the distortion of the shape of the drop, their asymmetry arises. In this case, pure force (dynamic fastening) is generated, which balances the opposing forces (static fastening, gravity and viscous forces). As a result, it was found that the highest forces can be generated at a contact angle of about 65.5 °.

    The intermediate result is that drops can easily overcome inclined surfaces, rising at a speed of 0.57 mm / s, while overcoming gravitational forces ( 4a ).

    Image No. 4

    Vertical movement of a drop.

    The movement of a drop on the ceiling (corresponds to image 4b ).

    In the demonstrated “mechanical-humidification” device, the maximum generated force was 2 μN (micronewton). The distance that a drop can cover (in any position) is limited only by the dimensions of the experimental device itself. An increase in the working surface will increase the distance traveled by the drop at the same speed.

    However, many of you will ask - what drops should there be for this beauty to work? Researchers also asked this question and tested their methodology on different liquids (water, isopropyl alcohol and mineral oil). Practical tests have shown that there is not much difference between the three options, and all of them are excellent for implementing the methodology under study.

    Demonstration of the simultaneous movement of many drops of different volume and, accordingly, size (corresponding to image 4c ).

    Researchers note that the presence of a critical angle as an important aspect of this technique allows you to sort drops. By tilting the setup at a certain angle, you can see that drops with a critical angle below the threshold value cannot move along the wave, while drops that show large maximum critical angles will be transported. Therefore, only drops of a certain size will move along with the wave, and the rest will slide off the inclined surface. When two drops combine, this sorting process is repeated, and the drop will be sorted based on its new size.

    And what about self-cleaning surfaces? This is a pretty useful property, isn't it? So, the researchers conducted observations of the droplets moving around the installation. They determined that these drops are able to clean the surface of contaminants.

    Next, scientists created a model of the contaminated surface by applying a large amount of calcium carbonate (CaCO 3 with a particle size <50 μm) to the surface of the PDMS film (covers the device).

    Turning on the traveling wave device, CaCO 3 particles did not disappear anywhere, but retained their position on the surface, which is associated with the transverse nature of the wave. Then, drops of water and isopropyl alcohol were applied to the contaminated surface. Drops move along the surface, thereby cleaning it from contamination.

    Cleaning contaminated surfaces with “running” drops (corresponds to image 4d ).

    However, there is a negative effect in this process. The presence of CaCO3 particles on the working surface led to an increase in resistance and a decrease in the effective droplet velocity to 20% of the wave velocity. But this did not stop the drops from cleaning the surface along their route.

    Scientists also fully understand the provocative moment of the narrowness of the use of traveler drops exclusively within the framework of an experimental setup. In the future, they plan to use sensitive surface topographies, which are characterized by mechanical deformation in response to external stimuli, such as light, magnetic fields, and temperature.

    Photosensitive liquid crystal polymers and elastomers are of particular interest to researchers. And all because of their precise spatio-temporal control, which allows wave-like movement on the surface with the help of structured or moving light sources or with constant illumination by self-shadowing.

    For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists and additional materials to it.


    First of all, I would like to note that the experiments conducted using the test setup developed by scientists are not yet a finished product or technology. This setup was used to demonstrate the studied dynamic droplet technique and self-cleaning function.

    In the future, scientists plan to use other technologies, in particular liquid crystal polymers and elastomers, which we spoke about earlier. This work is only confirmation that the technique of mechano-hydration has every right to exist, since it has advantages over other technologies whose goals are similar to this technique.

    One of the main areas where this miracle of scientific thought can be applied is microfluidics, that is, the diagnosis of liquids using small-sized devices. And the self-cleaning function can find its application in the creation of self-cleaning medical devices, marine sensors, windows, solar panels and even in the implementation of technologies for collecting dew.

    Whatever future this technology awaits, the very fact of its development speaks of the exceptional nature of human intelligence. Our brain is able to generate the most extraordinary, transcending ideas that are sometimes difficult to implement, but the result of this implementation confirms that nothing is impossible for us.

    Thank you for your attention, remain curious and have a good working week, guys! :)

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