In this paper, we studied the effect of surface wettability on the bubble tolerance of a piezoelectric membrane pump, by applying the super-hydrophilic or super-hydrophobic surface to the key elements on the pump. Wettability for the flow passage surface has a direct influence on the air bubbles flowing in the fluid. Based on the existing research results, we first analyzed the relationship between the flow passage surface of the piezoelectric pump and the bubbles in the fluid. Then we made three prototypes where pump chamber walls and valve plate surfaces were given different wettability treatments. After the output performance test, results demonstrate that giving super-hydrophilic treatment on the surface of key elements can improve the bubble tolerance of piezoelectric pump; in contrast, giving super-hydrophobic treatment will reduce the bubble tolerance.
I. INTRODUCTION
Over the decades of research, the piezoelectric membrane pump (referred to as piezoelectric pump) has achieved significant improvement in terms of its performance and stability. It has a wide range of application and broad prospects in the industries such as biomedical science, fine chemicals, fuel feeding and household appliance.1–4 However, in the commercialization of the piezoelectric pump, there are still many difficulties yet to be solved. One of the main problems encountered is that the piezoelectric pump is extremely sensitive to air bubbles. Once bubbles appear in the pump chamber, they would reduce the rate of fluid compression in the pump chamber and increase fluid flow resistance, then the output performance of the pump would drop sharply. Excessive stranded bubbles in the chamber would even prevent the pump from performing.5–9 The formation of bubbles has a variety of reasons. In addition to the external bubbles brought by the liquid, the internal microbubbles gather, fuse and grow up under the vibration of the piezoelectric vibrator, as well as the cavitation caused by the pressure change in the flow passage. These are the main sources of the bubbles in the piezoelectric pump. Researchers have been adopting a variety of effective methods to enhance the bubble tolerance of the piezoelectric pump, for instance, by changing the structure of the pump or improving the structure of the valve.
In 2001, Andersson et al10 successfully developed a valveless micropump which could convey liquid or gas. The valveless structure allowed the bubble to pass through the valve hole smoothly when conveying liquid. However, the overall output performance of the micropump is not satisfactory, because some of the bubbles stranded in the slow-flowing area of pump chamber and cannot be discharged quickly. In 2015, Ma et al11 designed the internal rib structures on the pump chamber and demonstrated that this structure can improve the bubble tolerance of the piezoelectric pump. In 2015, Dolžan et al12 designed a piezoelectric pump based on a centrally placed inlet port which leads directly into the center of the pumping chamber. It features self-priming ability and high-level bubble tolerance. In 2015, Chen et al13 studied the influence on the pump’s bubble tolerance by the wheel check valve. He concluded that the bubble tolerance can be improved by increasing the opening height of the wheel check valve and decreasing the air block probability.
The above methods to enhance the bubble tolerance from the structure perspective all have limitations. They can only be applied to the piezoelectric pump with a specific structure but cannot be commonly used. Meanwhile, these methods are only effective when a small amount of tiny bubbles in the pump. With the increase of one bubble volume in the chamber of piezoelectric pump, the adhesion between the bubble and the flow channel wall is enhanced, and finally the bubble cannot flow. In this case, the piezoelectric pump fails to work effectively. The wettability alteration method can change the adhesion strength between the surface and the bubbles in the liquid. Thus, we considered from the perspective of the material inside the pump. Based on the characteristics of surface wettability, we applied the super-hydrophilic/super-hydrophobic surface to the piezoelectric pump, and conducted an experimental research on the bubble tolerance of the pump. After the experiment, we proposed some methods to improve the output performance of the piezoelectric pump.
II. STRUCTURE AND WORKING PRINCIPLE
The structure of the piezoelectric pump developed in this study is shown in Fig. 1. It mainly consists of a pump body, a circular piezoelectric vibrator with a monomorph, an inlet check valve and an outlet check valve. The pump body is made from PMMA (Polymethyl Methacrylate), a highly transparent material, so that the internal situation of the pump chamber can be well observed. The circular piezoelectric vibrator – the driving element, is a circular metal substrate with a circular monomorph adhered to its one side. The inlet and outlet check valves, which are made from PET film, are in the shape of wheel structure. The valve plate is made from copper and processed by UV laser cutting.
When the AC voltage is applied to the circular piezoelectric vibrator, it will have periodically reciprocating vibration. The circular piezoelectric vibrator vibrating leads to periodic changes of the pump chamber volume. Under the coordination of the two check valves, the process of fluid suction and discharge is formed. The constant suction and discharge eventually achieve the fluid’s one-way flow, as shown in Figs. 2(a) and 2(b).
Working principle of the piezoelectric pump. (a) Suction process and (b) Discharge process.
Working principle of the piezoelectric pump. (a) Suction process and (b) Discharge process.
III. THE INTERACTION BETWEEN SOLID SURFACE AND BUBBLES
Many researchers studied the influence from the surface wettability on bubbles. Kitagawa et al have experimentally investigated the influence of wall surface wettability on the collective behavior of hydrogen microbubbles rising along a nearly vertical wall. They have shown that an increase in wall surface wettability leads to a decrease in the number of bubbles attached to the wall. That is to say, poor wettability significantly affects microbubble diffusion, while in the case of very good wettability, almost no microbubbles attach to the wall.14 The study of nanobubbles on hydrophilic and hydrophobic alumina surfaces has also shown the surface of modified hydrophobic Al2O3 was almost completely covered with nanobubbles whereas bubbles were hardly detected on hydrophilic alumina.15
To verify the influence of super-hydrophilic/super-hydrophobic surface on air bubbles in the liquid, in this study we prepared different kinds of surface by the coating method. The methods of preparing hydrophilic/hydrophobic surfaces are various, but we used the coating method because it is simple and fast. The hydrophilic coating material is silicon dioxide composite solution. And the hydrophobic coating material is silane composite solution. First, all the parts were cleaned in the ultrasonic cleaner and wiped afterwards. Then we sprayed the hydrophilic material on the surface of the parts and wiped the coating material evenly with a clean cloth. Finally, the parts were left in the air for 60 minutes until the coating surface was completely dried, to obtain a stable super-hydrophilic surface. The preparation method of super-hydrophobic surface was like the above. After coating with the hydrophobic material, a layer of nanoparticle silica film was formed on the surface of the parts, which led to super-hydrophobic effect. Contact angles were measured by contact angle measuring instrument. For our hydrophilic surface, it was 5° and for our hydrophobic surface, it was 155°.
We took three transparent PMMA glass containers and coated their inner walls respectively with super-hydrophilic material, super-hydrophobic material and nothing. The contact angles of the three surfaces are 5°, 155° and 78°. As shown in Fig. 3, the constant temperature distilled water mixed with air bubbles was injected into the three containers, and the bubble attachment on the bottom surface of the container was observed by using an image magnifier (model is VMS1510G). Observed phenomena are shown in Fig. 4. The bottom surface of the container without any treatment is easy to attach tiny round bubbles, and slight vibrations can cause the bubbles’ escape from the surface, as shown in Fig. 4(a). In the container with the super-hydrophilic treatment, the surface has a strong repulsion on bubbles, so bubbles are difficult to attach to the bottom of the container, as shown in Fig. 4(b). On the contrary, in the container with the super-hydrophobic treatment, the bottom adheres many irregularly shaped, flat air bubbles, and these bubbles are difficult to detach from the surface, as shown in Fig. 4(c).
Bubble attachment on the surfaces with different wettability. (a) No treatment, (b) Super-hydrophilic treatment, and (c) Super-hydrophobic treatment.
Bubble attachment on the surfaces with different wettability. (a) No treatment, (b) Super-hydrophilic treatment, and (c) Super-hydrophobic treatment.
The above experimental phenomena confirmed the conclusion of the other researchers. The surface with super-hydrophilic treatment cannot easily adhere bubbles, so it is conducive to the movement and discharge of bubbles; the surface with super-hydrophobic treatment tends to adhere the bubbles, so bubbles attached to the wall will not easily move or be discharged. From the above conclusion we can infer that, if we carry out super-hydrophobic treatment on the piezoelectric pump chamber wall and valve plate surface, the air bubbles entering the pump chamber are likely to stay in the chamber; conversely, if we carry out super-hydrophilic treatment on the chamber wall and check valve plate surface, we can improve the flow capacity of bubbles and reduce the probability of bubble blocking valve, thereby improving the bubble tolerance of the pump.
IV. EXPERIMENTAL TESTING
A. Making prototypes
Figure 5 is a photo of the prototype for the piezoelectric pump presented in this paper. Its external dimensions are 20mm x 20mm x 6mm. The diameter of the pump chamber is 12mm and the height is 0.05mm. The inner diameter of the inlet and outlet pipe is 1.2mm. The inlet and outlet check valves use the wheel valve structure and the valve plate is bonded with the pump chamber by high-strength UV glue.
To study the effect of the pump chamber wall (including pump body and piezoelectric vibrator) and the valve plate on the bubble tolerance of the piezoelectric pump, from the perspective of surface wettability, we gave the chamber wall and the valve plate surface different wettability treatments, so three prototypes were obtained after assembly. Details are shown in Table I.
Wettability treatment on prototypes.
Prototype . | Surface of pump chamber wall (contact angle) . | Surface of valve plate (contact angle) . |
---|---|---|
Prototype 1 | No treatment (78°) | No treatment (85°) |
Prototype 2 | Super-hydrophilic treatment (5°) | Super-hydrophilic treatment (5°) |
Prototype 3 | Super-hydrophobic treatment (155°) | Super-hydrophobic treatment (155°) |
Prototype . | Surface of pump chamber wall (contact angle) . | Surface of valve plate (contact angle) . |
---|---|---|
Prototype 1 | No treatment (78°) | No treatment (85°) |
Prototype 2 | Super-hydrophilic treatment (5°) | Super-hydrophilic treatment (5°) |
Prototype 3 | Super-hydrophobic treatment (155°) | Super-hydrophobic treatment (155°) |
B. Testing the device
To study the effect of the surface wettability on bubble tolerance of piezoelectric pump, we designed a piezoelectric pump performance test and set up the device which is shown in Fig. 6. The medium conveyed by the piezoelectric pump was distilled water. The water was heated and preserved to 54-60 °C by a thermostatic water tank. The height difference between the prototype and the testing liquid level was 95mm. The output flow rate and output pressure of the prototype were tested at the optimum operating frequency when the driving signal was a rectangular wave with 110Vpp. The output flow rate was measured by the weighing method, and the output pressure was measured by a manometer. During the performance test, when the cylinder drove the inlet tube of the pump out of and into the liquid surface, one air bubble was generated and entered the pump. The size of the bubble (about 0.015ml) was controlled by the time when the inlet tube was out of the liquid. The time interval between each bubble was constant. The output performance of the pump was tested after a group of 10 bubbles enter, and it was measured through the changes of the output flow rate and output pressure from different prototypes. Each prototype was measured 4 times and then calculated the average of the measured data.
C. Experimental results and discussion
The relationship between the output flow rate and the number of bubbles on the three tested prototypes is shown in Fig. 7. When the number of bubbles entering the pump chamber was zero, all the three prototypes achieved the optimum output flow rate, which were 15.8 ml/min, 17.5 ml/min and 13.5 ml/min respectively. However, with the increase of the number of bubbles entered, the output flow rate for the piezoelectric pump varied. On Prototype 1 which was without any treatment, the output flow rate decreased significantly with the increase of bubbles. On Prototype 2 with super-hydrophilic treatment, the output flow rate went down slightly when bubbles started to enter, but overall it tended to be stable. Therefore Prototype 2 showed a better bubble tolerance compared with Prototype 1. Prototype 3 with super-hydrophobic treatment, the output flow rate dropped sharply with the increase of the amount of bubbles and continued to go down until failing to perform. Likewise, the relationship between the output pressure and the number of bubbles on the three prototypes is shown in Fig. 8. It shows generally the same variation with output flow rate.
The relationship between the output flow rate and the number of bubbles.
The relationship between the output pressure and the number of bubbles.
From the above experimental phenomenon, we can see that, when giving super-hydrophilic treatment on the chamber wall and valve plate surface of the piezoelectric pump, the pump’s bubble tolerance can be improved effectively, compared with a pump without any treatment; when super-hydrophobic treatment is done on the pump chamber wall and valve plate surface, it is not conducive to the discharge of bubbles, therefore the pump’s bubble tolerance is greatly reduced.
When the bubbles entered the chamber, they flowed with the liquid from the inlet to the outlet along the shortest path between the two check valves. In this process, due to the vibration of the piezoelectric vibrator and the flap of the valve, a part of the bubbles were broken into smaller bubbles. Figures 9(a) and 9(b) present the bubble retention in Prototype 1. Most of these small bubbles were discharged with the liquid from the pump outlet, but some bubbles flowed to the peripheral area of the pump chamber. The liquid flow speed in this area was slow, so bubbles could easily gather here, merge, grow up and attach to the chamber wall. As the number of bubbles entered increased, these bubbles continued to grow, resulting in the output performance of the piezoelectric pump dropped down.
Bubble retention with the increasing bubbles. (a) 10 bubbles, (b) 80 bubbles, (c) 10 bubbles, (d) 80 bubbles, (e) 10 bubbles, and (f) 80 bubbles.
Bubble retention with the increasing bubbles. (a) 10 bubbles, (b) 80 bubbles, (c) 10 bubbles, (d) 80 bubbles, (e) 10 bubbles, and (f) 80 bubbles.
Figures 9(c) and 9(d) show the bubble retention in the Prototype 2. Bubbles here were easier to move in the pump chamber. Even in the peripheral area of the chamber, many bubbles did not attach to the chamber wall. They could be discharged along the outlet of the pump chamber, which ensured the stable output performance of the piezoelectric pump.
Figures 9(e) and 9(f) present bubble retention in Prototype 3. The bubbles entering the Prototype 3 diffused around the pump chamber and fused with the bubbles attached to the wall. When inputting bubbles into the pump continuously, the bubbles in the pump chamber became finely divided. They could hardly pass through the outlet valve and even be stuck in the outlet, eventually leading to a sharp decline in the output performance of the piezoelectric pump.
V. CONCLUSIONS
The bubbles retaining in the pump chamber drastically reduce the output flow rate and output pressure of the piezoelectric pump.
When the inner wall of the pump chamber and the valve plate surface are given super-hydrophilic treatment, this will increase the bubble flow capacity and reduce the possibility of bubble blocking check valve. Thus, this method can improve the bubble tolerance of piezoelectric pump. However, when the inner wall of the pump chamber and the valve plate surface are given super-hydrophobic treatment, this will cause bubble retention when bubbles flow through the check valve. It will hinder the bubble discharge and will reduce the bubble tolerance of the piezoelectric pump.
ACKNOWLEDGMENTS
This study is supported by the National Natural Science Foundation of China (No. 51406065).