Universal Plasma Jet for Droplet Manipulation on a PDMS Surface towards Wall-Less Scaffolds

Abstract

Droplet manipulation is important in the fields of engineering, biology, chemistry, and medicine. Many techniques, such as electrowetting and magnetic actuation, have been developed for droplet manipulation. However, the fabrication of the manipulation platform often takes a long time and requires well-trained skills. Here we proposed a novel method that can directly generate and manipulate droplets on a polymeric surface using a universal plasma jet. One of its greatest advantages is that the jet can tremendously reduce the time for the platform fabrication while it can still perform stable droplet manipulation with controllable droplet size and motion. There are two steps for the proposed method. First, the universal plasma jet is set in plasma mode for modifying the manipulation path for droplets. Second, the jet is switched to air-jet mode for droplet generation and manipulation. The jetted air separates and pushes droplets along the plasma-treated path for droplet generation and manipulation. According to the experimental results, the size of the droplet can be controlled by the treatment time in the first step, i.e., a shorter treatment time of plasma results in a smaller size of the droplet, and vice versa. The largest and the smallest sizes of the generated droplets in the results are about 6 µL and 0.1 µL, respectively. Infrared spectra of absorption on the PDMS surfaces with and without the plasma treatment are investigated by Fourier-transform infrared spectroscopy. Tests of generating and mixing two droplets on a PDMS surface are successfully achieved. The aging effect of plasma treatment for the proposed method is also discussed. The proposed method provides a simple, fast, and low-cost way to generate and manipulate droplets on a polymeric surface. The method is expected to be applied to droplet-based cell culture by manipulating droplets encapsulating living cells and towards wall-less scaffolds on a polymeric surface.

Keywords: droplet manipulation; plasma jet; surface modification.

Figure 1

An overview of the proposed method. (a) The process of surface modification by plasma jet. (b) The process of droplet generation and manipulation by airflow.

Figure 2

The experimental setup. (a) A photo of the experimental setup. (b) A illustrative diagram of the experimental setup.

Figure 3

Surface tensions and drag force on a droplet. (a) A droplet sits on a surface without jetted air. (b) The droplet is deformed and moved by the drag force $F_D$ from the jetted air.

Figure 4

Mechanism of the droplet generation and manipulation with sampled photos from experiments. (a) The process of the droplet generation from a source drop. (b) The process of the droplet manipulation with jetted air.

Figure 5

An example of droplet images taken by the top and the side cameras. The size of the droplet is estimated by the covering area and the height. (a) Top-view of a droplet. (b) Side-view of the droplet.

Figure 6

Examples of different size droplets generated and manipulated by different treatment times of plasma. The treatment times are (a) 0.08 s/mm. (b) 0.05 s/mm. (c) 0.04 s/mm. (d) 0.029 s/mm.

Figure 7

The relation of droplet sizes and treatment time of plasma.

Figure 8

Mixing test. (a) The mixing system and the processes of the mixing test. (b) Comparison of experimental values and expected values in the mixing test.

Figure 9

The spectra of PDMS surface with and without plasma treatment is measured with a FTIR spectrometer. A weak broad peak is observed in the range of 3000 and 3700 cm${}^{-1}$.

Figure 10

Aging effect of plasma treatment. (a) The measurement of contact angle of treatment time of plasma of 0.08, 0.05 and 0.029 s at aging time of 0, 60 and 120 min. (b) Aging effect of different treatment time of plasma. (c) The relation of contact angle and droplet size.

Figure 11

Droplet size is found decreased with the increase number of manipulations on the same plasma-treated path.

Figure 12

Repeated droplet generation with different plasma treatment. (a) A photo of the first droplet. (b) A photo of the fourth droplet. (c) No droplet is generated after nine times of generation. (d) The relation of the droplet size and times of generation at different treatment time of plasma.