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JMST Advances

, Volume 1, Issue 1–2, pp 41–47 | Cite as

Water permeable flow of polydimethylsiloxane controlled by physicochemical treatment

  • Gyungmok Nam
  • Sungmin Park
  • Sangheon Park
  • Young Choi
  • Seungpyo Woo
  • Sang-Hee YoonEmail author
Letter
  • 148 Downloads

Abstract

For polydimethylsiloxane (PDMS), a formation of interconnected pores and an addition of Silwet L-77 are, respectively, made as physical treatment and chemical one to change the material properties involved in the water permeable flow through PDMS. Here, we investigate a change in the water permeable flow through PDMS induced by physicochemical treatment. 28 kinds of physicochemically treated PDMS (pc-PDMS) blocks having different pore sizes of 50–500 µm and different Silwet L-77 concentrations of 0.0–8.0 wt% are prepared using a pressure-assisted compaction and NaCl particle-leaching technique. The values of mass flow rate and flow delay are obtained from pressure-driven water flow through pc-PDMS blocks as indexes for characterizing their water permeable flow. Our physicochemical treatment successfully controls the water permeable flow through PDMS, which means that pc-PDMS can be used for the development of powerless microfluidic regulators for aqueous chemicals in micro-total analysis systems (µ-TAS).

Keywords

Microfluidic regulator Physicochemical treatment Polydimethylsiloxane Water permeable flow 

1 Introduction

A µ-TAS technology offers numerous advantages of precise reagent control, short reaction time, low cost, etc., over-existing analytical instruments used in chemistry and biomedical engineering [1]. In a recent effort to simplify the configuration of µ-TAS, there is a continuously increasing demand for the development of powerless (or passive) components that control the mixing amount and mixing sequence of aqueous chemicals in the µ-TAS. Considering that PDMS, of all the polymers, has been prevailingly used for prototyping or fabricating µ-TAS, diverse physical and chemical treatments of PDMS have been made to develop PDMS-based aqueous chemical flow-controlling components integratable to the µ-TAS [2, 3]. To be more particular, a variety of physical treatments such as wrinkle creation [4], pore formation [5], etc., have been mainly introduced to find new processing methods for PDMS except soft lithography and its modifications. Extensive chemical treatments (e.g., UV/plasma irradiation [6], surfactant addition [7], etc.) have been made in response to the strong hydrophobicity of PDMS. An in-depth understanding on the nature of water permeable flow through pc-PDMS is critically important for finding an optimal combination of physical treatments and chemical ones, and furthermore for the successful development of PDMS-based water (or aqueous chemicals) flow-controlling components in µ-TAS. Much, however, remains unclear about the parameters involved in water permeable flow through pc-PDMS. This leads to the need for quantitative measurement of a change in the water permeable flow through PDMS induced by physical, chemical, and physicochemical treatments.

Here, we prepare three groups of bulk-treated PDMS blocks—physically treated PDMS (p-PDMS), chemically treated PDMS (c-PDMS), and pc-PDMS—using a pressure-assisted compaction of NaCl particles and their leaching in water together with a surfactant addition to intact PDMS. The p-PDMS is a porous PDMS block within which interconnected pores having different diameters of 50, 125, 250, and 500 µm are formed; the c-PDMS is a surfactant-added PDMS block in which Silwet L-77 is blended with intact PDMS at different concentrations of 0.0, 0.1, 0.5, 1.0, 2.0, 4.0, and 8.0 wt%; the pc-PDMS is a porous and surfactant-added PDMS block, as shown in Fig. 1. For three groups of bulk-treated PDMS blocks, we perform a pressure-driven flow of water through them to characterize their change in the water permeable flow caused by physical, chemical, and physicochemical treatments. The values of mass flow rate and flow delay (time) are determined from the pressure-driven flow through each PDMS block as characteristic indexes for water permeable flow by measuring the time profile of the mass of water passed through each PDMS block per unit time at a constant pressure of 1 kPa.
Fig. 1

Change in the water permeable flow through PDMS induced by physical, chemical, and physicochemical treatments

Our work has the flowing features in characterizing a change in the water permeable flow through PDMS induced by physical, chemical, and physicochemical treatments. First of all, to the best of our knowledge, this is the first study systematically addressing the effects of physical, chemical, and physicochemical treatments on the behavior of water permeable flow through PDMS. Second, all experiments are carried out at the conditions very similar to those of µ-TAS in real engineering. In detail, water is used as a substitute of aqueous chemicals and a constant pressure of 1 kPa is the differential pressure value of general micropumps [8]. Last but not least, our bulk treatment methods for PDMS is highly compatible to the common fabrication methods used in µ-TAS, which means that this approach can be directly used for the development of powerless and integratable flow-controlling units for aqueous chemicals in µ-TAS. Our approach, therefore, leads to an explosive use of p-, c-, and pc-PDMS in µ-TAS areas and results in a better understanding of water permeable flow through PDMS.

2 Materials and methods

2.1 Preparation of bulk-treated PDMS

The bulk-treated PDMS blocks (i.e., p-, c-, and pc-PDMS) were prepared using a pressure-assisted compaction of water-soluble particles (i.e., NaCl particles) and their leaching in deionized water (DI water), together with an addition of Silwet L-77 (Helena Agri-Enterprises) to intact PDMS, as shown in Fig. 2a. First of all, natural sea salt was mechanically ground using a blender and then sifted with five sorts of sieves having average mesh sizes of 50, 125, 250, 500, and 750 µm, therefore, being assorted into five kinds of NaCl particles having different sizes of 50–125 µm (here, indicated as ‘50 µm’), 125–250 µm (‘125 µm’), 250–500 µm (‘250 µm’), and 500–750 µm (‘500 µm’). Next, a biocompatible surfactant, Silwet L-77, was blended with 10:1 (base to curing agent weight ratio) PDMS (Sylgard 184, Dow Corning) at different concentrations of 0.0, 0.1, 0.5, 1.0, 2.0, 4.0, and 8.0 wt%. A mixture of Silwet L-77 and PDMS was poured into the compacted NaCl particles which were located within a container having a lot of small drain holes. Excessive PDMS was removed by applying a pressure of 10 MPa to secure the interconnection of NaCl particles and then thermally cured for 24 h at 65 °C. The thermally cured mixture was dipped in a warm aqueous Silwet L-77 solution at the critical micelle concentration to completely dissolve the NaCl particles, followed by thermal drying for 1 h at 100 °C.
Fig. 2

Experimentals. a Preparation of the bulk-treated PDMS, especially pc-PDMS, and b experimental setup for the characterization of water permeable flow through bulk-treated PDMS

2.2 Characterization of water permeable flow

The nature of a water permeable flow through bulk-treated PDMS was characterized by measuring the mass (or weight) of water passed through the bulk-treated PDMS block per unit time at a constant pressure. The value of applied pressure was determined as 1 kPa, considering that of differential pressure of general micropumps [8]. We introduced a constant pressure-driven water flow to the bulk-treated PDMS block using a small-bore tube method in which the height of water was kept as 100 mm, corresponding to a pressure of 1 kPa, by draining water given in excess at a continuous supply of water, as shown in Fig. 2b. In this measurement, a cylinder-shaped bulk-treated PDMS block with a diameter of 12 mm and a height of 10 mm was placed at the outlet channel of the small-bore tube apparatus. The weight of water passed through each bulk-treated PDMS block was measured with a precision scale (CAY-120, CAS).

3 Results and discussion

The time profile of a pressure-driven water flow through bulk-treated PDMS blocks was obtained to characterize a change in the water permeable flow of PDMS caused by three kinds of bulk treatments (i.e., physical, chemical, and physicochemical treatments). The most obvious trend in the time profile was that there were three distinguishing zones (see Fig. 1). When a water flow was introduced to each of the bulk-treated PDMS blocks, the water flow delayed until the pressure (here, 1 kPa) for displacing water in the interconnected pores of the bulk-treated PDMS block balanced shear stress near the wall (zone I, flow delay zone). Once water flowed through the interconnected pores, the water filled in all the interconnected pores and started to be fully developed, thus having a transient mass flow rate in the zone II (i.e., transient zone). In the zone III (i.e., steady zone), the water flow was stabilized and had no temporal change in its mass flow rate.

To investigate the effects of physical treatment on the water permeable flow of PDMS, the mass of water which passed through p-PDMS blocks having different pore sizes of 50–500 μm and an identical Silwet L-77 concentration of 1.0 wt% at a pressure of 1 kPa was measured, as shown in Fig. 3a. A 50-μm pored p-PDMS block had no water flow, whereas the others had a continuous water flow after flow delay. For the p-PDMS blocks, the mass flow rate, corresponding to a slope of the time profile of water permeable flow, was in proportion to the pore size. The value of flow delay was inversely proportional to the pore size. The proportionality between mass flow rate and pore size is quite natural because the interconnected pores play as a bundle of long, cylindrical microchannels for water permeable flow. From the measurement on flow delay, a formation of interconnected pores within intact PDMS is shown to increase the hydrophobicity of PDMS, which is well matched with the Cassie-Baxter model [9]. A decrease in the surface tension between water and PDMS owing to Silwet L-77 diffusion during water permeable flow is, however, enough to overcome an increase in the hydrophobicity of PDMS caused by a formation of interconnected pores (except 50-µm sized pores).
Fig. 3

Time profiles of water permeable flow measured from p-PDMS (a) and c-PDMS blocks (b). The p-PDMS blocks have different pore sizes of 50–500 µm and a constant Silwet L-77 concentration of 1.0 wt%; the c-PDMS blocks have a pore size of 125 μm and different Silwet L-77 concentrations of 0.0–8.0 wt%. From the time profiles, the values of mass flow rate and flow delay are determined as two characteristic indexes

The dependence of the water permeable flow of PDMS on our chemical treatment was also characterized. To serve this purpose, we measured the mass of water passed through c-PDMS blocks with an identical pore size of 125 μm and different Silwet L-77 concentrations of 0.0–8.0 wt% at a pressure-driven water flow at 1 kPa. There were two noticeable points, as shown in Fig. 3b. The mass flow rate was not affected by chemical treatment (or Silwet L-77 concentration), but the delay in a water permeable flow was varied significantly by adjusting the Silwet L-77 concentration in an inversely proportional manner. The c-PDMS blocks have the same interconnected pores therein, so that an invariance in mass flow rate is quite reasonable. In addition, there is the accumulation of Silwet L-77 in the interface between water and c-PDMS through diffusion during water flow, which leads to a temporal change in the surface tension. The flow delay, therefore, decreases with increasing Silwet L-77 concentration. These can explain a change in the water permeable flow of the c-PDMS blocks.

Next, we explored a change in the water permeable flow of PDMS induced by our physicochemical treatment, as shown in Fig. 4. Since the physicochemical treatment was a combination of physical treatment and chemical treatment, the following results were not surprising. The value of mass flow rate was proportional to pore size but insensitive to Silwet L-77 concentration; the value of flow delay was in an inverse proportion to both pore size and Silwet L-77 concentration. In detail, the value of mass flow rate increased from 0.00 g/s to 1.80 g/s and that of flow delay decreased from ∞ seconds to 5 s when pc-PDMS blocks had an increase in their pore size from 50 μm to 500 μm but an identical Silwet L-77 concentration of 1.0 wt%. A variation in the Silwet L-77 concentration from 0.0 to 8.0 wt% of pc-PDMS blocks with an identical pore size of 250 μm led to no change in mass flow rate (i.e., no water flow for Silwet L-77 concentrations of 0.0–0.1 wt%, 0.22 ± 0.01 g/s for Silwet L-77 concentrations of 0.5–8.0 wt%) but had a decrease in flow delay from 300 s to 5 s. The measurements show that the water permeable flow of PDMS can be controlled by the application of our physicochemical treatment. The physical treatment involved in a formation of interconnected pores determines the mass flow rate of a water permeable flow and the chemical treatment (i.e., an addition of Silwet L-77), with the aid of physical treatment, adjusts the flow delay in the water permeable flow.
Fig. 4

Effects of physicochemical treatment on the water permeable flow of PDMS. Flow delay (a) and mass flow rate (b) of the pc-PDMS blocks as a function of pore size and surfactant concentration. Top insets show the values of flow delay and mass flow rate at a pore size range of 50–500 μm and a constant Silwet L-77 concentration of 1.0 wt%; right ones display those at a constant pore size of 250 μm and a Silwet L-77 concentration range of 0.0–8.0 wt%

Staining experiments were performed with pc-PDMS blocks having an identical pore size of 250 μm and different Silwet L-77 concentrations of 0.0–8.0 wt% to visualize the mass flow rate and flow delay of a water permeable flow through PDMS controlled by our physicochemical treatment. Figure 5 depicts the image of a water permeable flow, stained in blue, through the pc-PDMS blocks when an identical water head of 100 mm is applied to them. A continuous water permeable flow was made from right to left and then the mass flow rate was almost same after the water permeable flow through pc-PDMS blocks was stabilized. This is well agreed with the experimental results on physical treatment and chemical one (see Fig. 4). Considering that our physicochemical treatment is highly compatible to the conventional fabrication methods for PDMS, our physicochemical treatment of PDMS is believed to be an effective approach for the development of microfluidic regulators that can control the mass flow rate and flow delay of aqueous chemicals in μ-TAS in a powerless way. Furthermore, this method might contribute on the simplification of a μ-TAS configuration.
Fig. 5

Visualization of the water permeable flow through pc-PDMS blocks having a pore size of 250 µm and different Silwet L-77 concentrations of 0.0–8.0 wt% after 0 (a), 1 (b), 50 (c), 100 (d), 200 (e), and 300 (f) seconds of water loading corresponding to a pressure of 1 kPa

4 Conclusions

We have characterized the effects of physical, chemical, and physicochemical treatments on the water permeable flow through PDMS, therefore, providing critical hints for the development of powerless fluidic circuits that can regulate the mixing sequence and mixing ratio of aqueous solutions in μ-TAS. To change the nature of a water permeable flow through PDMS, a formation of interconnected pores and an addition of Silwet L-77 to the bulk of intact PDMS as physical treatment and chemical one, respectively. A set of 12-mm-diameter and 10-mm-high bulk-treated PDMS blocks having diverse pore sizes of 0–500 µm and different surfactant concentrations of 0.0–8.0 wt% was measured to have a mass flow rate of 0.00–2.11 g/s and a flow delay of 0–250 (or ∞) seconds at a pressure of 1 kPa, which means the water permeable flow of PDMS can be controlled by our physicochemical treatment in a passive way. An extrapolation of the findings to microfluidic devices will help us to achieve high level of integration within µ-TAS and also the simplification of a μ-TAS configuration.

Notes

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT & Future Planning (NRF-2017R1A2B4010300).

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Copyright information

© The Korean Society of Mechanical Engineers 2019

Authors and Affiliations

  • Gyungmok Nam
    • 1
  • Sungmin Park
    • 1
  • Sangheon Park
    • 1
  • Young Choi
    • 1
  • Seungpyo Woo
    • 1
  • Sang-Hee Yoon
    • 1
    Email author
  1. 1.Department of Mechanical EngineeringInha UniversityIncheonRepublic of Korea

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