Water permeable flow of polydimethylsiloxane controlled by physicochemical treatment
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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).
KeywordsMicrofluidic regulator Physicochemical treatment Polydimethylsiloxane Water permeable flow
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 . 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 , pore formation , 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 , surfactant addition , 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.
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 . 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
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 . 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.
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.
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.
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).