In order to simulate the moisture absorption performance of clothing fiber in textiles, an experimental platform that can manipulate and observe mesoscopic liquid was designed. This experimental platform can compile an executable program and upload it to Arduino development board to control the pulse signal of stepper motor, and then make the stepper motor and digital microscope to achieve micro-scale high-precision spacing control and observation. The change of droplet morphology was observed by controlling the fiber spacing. It was found that the change of droplet morphology was related to the fiber radius and droplet volume. When the fiber radius was 0.25mm, the morphology of small droplets (less than 1.2μL) was reversible with the increase or decrease of the spacing. However, the morphology of the large droplets (greater than 1.2μL) showed hysteretic behavior with the change of fiber spacing. The research in this paper provides a reference for the manufacture of fiber textiles and the manipulation or transportation of small-scale liquids.

Keywords: micro-liter volume droplet; fiber tube; stepper motor; hysteresis behavior; droplet bridge; barrel-shaped droplet

1 Experimental System Architecture Design

1.1 Experimental Equipment

The experimental platform mainly consists of a computer PC, fiber tubes, Arduino development board, micropipette, EzM-42-A-D stepper motor, driver, digital microscope, etc. (see Figure 1).

1.2 Architecture Design

This experimental system connects the Arduino development board with the driver, and then connects the stepper motor with the driver. The stepper motor controls the experimental materials on the mechanical device, while the Arduino sends pulse signals that are amplified by the driver, driving the stepper motor to achieve precise manipulation of the experimental materials in a Windows environment by inputting commands through the serial port (see Figure 2).

1) Arduino UNO Development Board

The selected development board model is Arduino UNO, using microcontroller ATmega328P. It has a 16MHz quartz oscillator, power interface, USB interface for computer program loading, 14 digital I/O pins: 6 can be used as PWM output pins, and 6 can be used to read analog signal pins, reset button, and supports online serial programming).

2) Pulse Signal Generation

Using Arduino IDE software to write program code to input into the Arduino development board to control the pulse signal of the stepper motor. The pulse signal is a hardware allocation scheme. Since the pulse current generated by the Arduino chip is too small, it is generally not directly connected to the stepper motor, using a driver connected to the Arduino development board to amplify the pulse current. This reduces the load on the development board, and the direction signal is sufficient to support the TB6600 driver to allocate pulses to control the stepper motor.

3) Stepper Motor

This mechanical system connects the Arduino development board with the driver, and then connects the stepper motor with the driver. The stepper motor controls the spacing of experimental materials on the mechanical device. The stepper motor uses the EzM-42-A-D stepper motor (rated current: 2A, rated voltage: 12V, holding torque: 0.5N·m, step angle: 1.8° accuracy, effective stroke: 60mm, repeat accuracy ±0.5μm). The driver is the Toshiba TB6600 driver. The reasons for using a stepper motor are threefold: first, it can precisely control the spacing between fibers. Second, it can achieve higher torque at low speeds, making the spacing change constant and controllable. Third, it is an open-loop control with a high cost-performance ratio.

2 Implementation and Application

2.1 Experimental Steps

(1) Connect the circuit, open the Arduino IDE serial monitor, debug the pulse and stepper motor, clean the fiber tube with acetone and fix it parallelly on the mechanical platform.

(2) Connect the digital microscope to the PC, open the camera software, and focus to clearly observe the fiber tubes (radius of 0.25mm).

(3) Use Arduino IDE to control the spacing of the fiber tubes, place droplets of constant volume (approximately 0.5~10μL of water or oil) between the fiber tubes using a micropipette, start the program, and change the spacing between the two fiber tubes by ΔD=0.05mm (spacing adjustable), recording the shape changes of the droplets from the top and side with the digital microscope.

(4) Change the liquid volume and repeat the above steps.

(5) Analyze and process the acquired images. When the liquid volume is small, the precision of the digital microscope (1mm) is not sufficient, so the images are processed using ImageJ software. The diameter of the cylindrical fiber is used as a reference to obtain various distance and length values.

2.2 Experimental Results

To simulate the influence of the tightness of clothing fibers on capillarity, the shape changes of the liquid can be observed by continuously increasing and decreasing the spacing between fiber tubes. In the experiment, the droplet volume changes ranged from 0.1~4.0μL. It was found that non-volatile liquids placed on two parallel rigid fibers exhibited three equilibrium shapes (Figure 3): droplet bridge shape, where the liquid completely envelops both fibers; barrel-shaped droplet, where the liquid bridge wets the sides, partially wrapping both fibers; and long column shape.

When the fiber radius r=0.25mm, the droplet volume is divided into two regions: small volume droplets (approximately less than 1.2μL) and large volume droplets (approximately greater than 1.2μL). Vc≈1.2μL is the critical volume, and the droplet morphology shows distinctly different phenomena with the change of spacing. Taking volumes of 1.0μL and 3.8μL as examples (as shown in Figure 4).

For small volume (V=1μL) liquid (Figure 4(a)), as the spacing decreases, the droplet smoothly elongates into a liquid column. Conversely, as the spacing increases, the liquid column reverts to a droplet bridge. Both processes are reversible, and the critical spacing values for the transitions are the same (dimensionless distance). The critical distance values can be calculated through the curvature radius r of the liquid-gas interface, as shown in Figure 5, which illustrates the cross-section of the droplet, where the curvature r is represented.

When the liquid volume is relatively large (V=3.8μL) (Figure 4(b)), it can be observed that the barrel-shaped droplet suddenly diffuses into a liquid column at a certain critical distance DDC. Conversely, when the spacing is increased, the liquid column becomes rounder and shorter, and at a certain critical distance DCD, it suddenly shrinks into the shape of a droplet bridge. The values of DDC and DCD are not equal in this process, indicating that the transition from one state to another has hysteresis. This is the hysteresis phenomenon of large volume droplets.

The critical volume value is not fixed; it is related to the radius, spacing, and volume. Figure 6 depicts the relationship between the critical volume Vc and D at a given radius. It can be seen that fibers with a larger radius have a larger Vc, and Vc rapidly increases with the increase of distance. Each curve can be divided into two regions: when VVc, the liquid morphology is a barrel-shaped droplet.

The hysteresis phenomenon of large volume droplets may be related to surface energy and contact angle hysteresis. Specifically: (1) The minimum surface energy of the droplet determines its optimal morphology. The stability of the liquid-gas interface follows the principle of minimum energy, and the stable form of the liquid interface is its minimum energy state. The surface energy of large volume droplets is greater than that of small volume droplets, hence their morphological changes are more complex. (2) Considering inertial effects, the process of droplets moving as the spacing changes is a non-static process. When droplets expand and contract between two fibers, the contact angle in the forward direction is not equal to that in the backward direction, leading to an unbalanced surface tension effect at the three-phase contact line of the droplet due to the unequal contact angles. The surface tension imbalance of large volume droplets is more significant. (3) As the spacing between fibers changes, the droplet “slides” between the fibers, resulting in corresponding changes in the half-filled angle β (see Figure 7) on the liquid-solid contact line and the curvature of the liquid-gas interface. Small volume droplets undergo a transition when β=π, while large volume droplets begin to change morphology before reaching β. This is related to capillary pressure and hydraulic resistance.

Research on the hysteresis phenomenon of large volume droplets is limited, and its mechanism is not yet clear, thus requiring further in-depth study.

3 Conclusion

To analyze the moisture absorption and conduction performance of fiber tubes in textiles, this paper designed a manipulation system based on the Arduino platform. Using Arduino IDE software, a clear and easily iterated modular program was written, with the code input into the Arduino development board to control the pulse signal of the stepper motor. The hardware allocation of the pulse signal connects the driver with the driver, thereby enabling precise automatic control of the spacing between fiber tubes and observing the wetting and capillary phenomenon of liquids between the fiber tubes. The experiment simulates the moisture absorption and conduction of fibers, providing a reference for the development of moisture-absorbing and sweat-wicking fabrics.

References

Fund Project: National Natural Science Foundation Regional Fund (11665007); Guangxi Natural Science Foundation (2018GXNSFAA138190); Guangxi University Young Teacher Basic Ability Improvement Plan Project (2018KY0085); Guangxi Key Research and Development Plan (桂科AB19050003).

Corresponding Author: Zhang Miaojing, Female, Professor, mainly engaged in physics education and theoretical physics, [email protected].

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