**3. Basic overview of fabrication techniques**

#### **3.1. Membrane fabrication**

Design and fabrication are the initial steps to work with microfluidic devices. Different types of fabrication techniques include photolithography, electron lithography, hot embossing and injection molding, etc. Photolithography is a common technique when feature sizes larger than 1 μm are desired. The nanoscale feature can also be fabricated by e-beam lithography

**Figure 1.** Schematics of (a) membrane filtration where feed is wastewater and permeate is the clean water and (b)

The impact of hydrodynamic flow on biofilms is the large time-dependent deformations that can result in nonlinear phenomena. An example of such phenomena is the bacterial streamer. Streamers form in flowing water and attach to the surface by the upstream "head" while the downstream "tail" can oscillate [6, 10, 30–32]. Streamers in a microfluidic system are typically tethered at one end to the pillar walls while the rest of the body is suspended in the downstream direction. Their filamentous structure can extend significantly with the flow [6, 33, 34]. Drescher et al. [35] revealed that streamers can cause a sudden and rapid clog in the fluid flow system in comparison with the biofilm attached to the surface. Surface hugging biofilms have a very modest effect on the flow rate whereas; streamers can drastically decrease the flow rate

Rusconi et al. [36] reported streamer formation in the microfluidic channel under laminar flow conditions. They observed formation of a single streamer in the middle of the channel connecting the inner corners of the channel. They also claimed that secondary flows in the curved edge of the channel were responsible for the location of the streamer, which was located at the mid-plane. They further investigated the streamer formation behavior by changing the radius of the curvature of a zigzag microchannel and discovered that streamer formation depends on

Valiei et al. [6] observed streamers through the height of the channel with 50 × 8 array of micro-pillars and mentioned it as a 'web' of the streamers. They claimed that flow rate has a significant impact on the number of streamer formation. A higher number of streamer formations was reported in the middle of channel height. **Figure 2** shows the formation of bacterial streamers in a microfluidic device with an array of micropillars. The white arrow indicates the

where the minimum resolution could go down to 10 nm [28, 29].

**2.3. Bacterial streamer due to biofouling**

the geometric angle of microchannel [37].

in a very short period [31].

microfluidic filtration mode.

296 Microfluidics and Nanofluidics

Membrane process is an emerging separation technology. The membrane itself is the heart of a membrane process. It can be classified as polymeric and inorganic, porous and dense, isotropic and anisotropic, hydrophilic and hydrophobic, etc. **Figure 3** gives an overview of types and preparation process of the polymeric membranes. Phase inversion (phase separation) and track etching are the most widely used techniques for the preparation of porous membranes [38].

In phase inversion process method, the polymer is transformed in a controlled manner from liquid to solid state by changing the thermodynamic state of the polymer, solvent and the solution [38, 39]. Symmetric porous phase inversion membranes are made by using water vapor as the coagulant. For making asymmetric membranes by phase inversion temperature increase and a liquid nonsolvent is used to precipitate the polymer (**Figure 3**). In track etching method, a high energy particle radiation is applied to the polymeric film, to damage the polymeric matrix and create tracks. By etching the polymeric material along the track uniform cylindrical

**Figure 3.** Preparation methods of polymeric membrane.

pores can be obtained. Dense membranes (symmetric and asymmetric) are mainly synthesized by solution casting and interfacial polymerization of two monomers on a substrate. A detailed explanation of membrane preparation techniques is available in the literature [38].

> A summary of different types of pressure-driven membrane processes with their fabrication technique, separation principle, pore morphology, pressure and flux ranges are given in the **Table 1**. Scanning electron microscopy (SEM) images of different types of membranes are

> **Figure 4.** (a) Surface SEM image of (a) a phase inversion porous membrane, (b) cross-sectional SEM image of an isotropic phase inversion membrane, (c) cross-sectional SEM image of an anisotropic phase inversion membrane, (d) surface SEM image of thin film composite (TFC) dense membranes, (e) cross-sectional SEM image of a TFC membrane, and (f) cross-

Microfluidic Membrane Filtration Systems to Study Biofouling

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There are many types of fabrication techniques available for making micro/nano devices such as photolithography, etching, soft lithography, hot embossing, injection molding, E-beam lithography, and micro-stereolithography. Photolithography and etching are two popular fabrication techniques. Soft lithography is a well-known method for microfabrication. McDonald et al. [42] fabricated microfluidic system with PDMS by a soft lithography technique to make 20–100 μm microfluidic structure. This technique has also worked well on hydrogel polymers (calcium alginate) to fabricate microfluidic network of 100 μm wide and 200 μm deep and 25 × 25 μm cross-section [43]. A complex structure with feature sizes larger than 20 μm can be achieved by using rapid prototyping [44]. The fabrication of 500–2000 μm diameters and 200–1000 μm height cylindrical columns [45] is possible by hot embossing technique. A schematic diagram of a microfluidic device is shown in **Figure 5**. This device is used to observe the biofilm behavior and the change of hydrodynamics of the fluid flow through the channel [6]. The chip has one inlet and one outlet and is made by traditional photolithography using

presented in **Figure 4**.

**3.2. Microfluidic device fabrication**

sectional SEM image of a TFC membrane.

polydimethylsiloxane (PDMS).


**Table 1.** Summary of different types of pressure-driven membrane processes [38–41].

**Figure 4.** (a) Surface SEM image of (a) a phase inversion porous membrane, (b) cross-sectional SEM image of an isotropic phase inversion membrane, (c) cross-sectional SEM image of an anisotropic phase inversion membrane, (d) surface SEM image of thin film composite (TFC) dense membranes, (e) cross-sectional SEM image of a TFC membrane, and (f) crosssectional SEM image of a TFC membrane.

A summary of different types of pressure-driven membrane processes with their fabrication technique, separation principle, pore morphology, pressure and flux ranges are given in the **Table 1**. Scanning electron microscopy (SEM) images of different types of membranes are presented in **Figure 4**.
