Capillary Assembly

Whereas the mechanism leading to particle immobilization was known for a long time, the parameters controlling particle deposition are numerous and their origin is not yet clear. - In the following section, we propose an overview of the main parameters experimentally accessible to control the deposition process, as well as a qualitative study of their influence on the deposition mechanism. 

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The membrane technology has been tested in microfluidic devices. Normally a membrane is mounted between two chips, which make a microchannel, and fluid is allowed to pass through the membrane channel. Some papers are available on this method, which are discussed here. Hisamoto et al. [61] reviewed the application of capillary assembled microchips on PDMS as an online... 

Henares et al. [210] developed a multiple enzyme linked immunosorbent assay (ELISA) chip by using capillary-assembled microchip (CAs-CHIP), which involved simple embedding of a 2 to 3 mm length of square capillaries possessing valving and immunoreaction functions into the microchannels fabricated on a PDMS substrate. The authors immobilized different anti-IgGs... 

Hisamoto, H., Nakashima, Y., Kitamura, C., Funano, S.I., Yasuoka, M., Morishima, K., Kikutani, Y., Kitamori, T., Terabe, S., Capillary-assembled microchip for universal integration of various chemical functions onto a single microfluidic device. Anal. Chem. 2004, 76, 3222-3228. 

In general, there are two approaches to assemble nanostructured materials, namely, physical assembly and chemical assembly. Physical assembly techniques are based on the assembly of nonfunctionalized nanoparticles on surfaces by physical forces, which include convective or capillary assembly,3,4 spin coating,5 and sedimentation.6 The physical assembly of nanoparticles generally results in relatively simple, closely packed two- or three-dimensional particle arrays. In addition, the physically assembled nanoparticle structures lack long term stability because they were deposited at relatively low surface pressures.7... 

The first multiple-capillary viscometer was designed by the Viscotek Company (Houston, TX, U.S.A.) [7]. It is represented in Fig. lb. It is composed of four identical capillaries, assembled as a bridge. Here, the difference is generated by the bridge itself as a differential viscometer. The central DPT provides a differential viscosity signal DP and the second, DPT, provides inlet pressure IP. The intrinsic viscosity [17] can be calculated by the formula... 

FIGURE 20.6 A typical coaxial capillary assembly for focusing flows. 

Hisamoto H, Yasuoka M, Terabe S (2006) Integration of multiple-ion-sensing on a capillary-assembled microchip. Anal Chim Acta 556 164M70... 

In 2004, Hisamoto and coworkers [19] have presented a simple approach, called capillary-assembled microchips (CAs-CHIP), for assembling a commercial square fused-silica capillaiy into a PDMS microfluidic device. The capUlaiy could be completely functionalized off-chip and cut into required size and then integrated raito a chip without any solution leakage (as shown in Fig. 3). As many methods for surface modification of capillary have been well established, the CAs-CHIP method offered a way to fabricate different microfluidic devices having various functions for analytical applications including sample pretreatment, biochemical sensors, and so on. 

Now, slide the ring from the watch glass and set up the culture as described in section 18.2.2.3. Finally, if necessary, suck off more fluid with the mouth capillary assembly to ensure that the graft site is totally dry and appears closed. 

The capillary module consists of a large number of capillaries assembled together in a module, as shown schematically in figure Vni -11. The free ends of the fibers are potted with agents such as epoxy resins, polyurethanes, or silicone rubber. 

Figure 2. Self-assembly processes for different particle sizes. 100 pm diameter beads were assembled on a structured template (bottom left, scale bar 1 mm) by gravitational assembly. Vibration of the template introduced sufficient mobility for the particles to move and be captured in holes in the template. Beads with 500 nm diameter were assembled from suspension in a meniscus that slowly moved over a structured carrier (bottom right, scale bar 10 pm) during capillary assembly. The capillary forces dragged the particles into shallow recesses and maintained them in position during drying.

Sdf-Assembly. Gravitational ass bly was performed on silicon tinplates with 60 pm de holes defined through standard photolithography that were coated with a perfluorodecyltTichlorosilane monolayer. Particles were d x>sited on the coated tonplat which was then placed in a sealed ceU. Ihe entire setup was excited at 60-100 Hz so that the amplitude was just sufficient to fluidize the particles. When all holes in the tinplate were filled with particles, the template was removed, and excess particles were picked up by a carrier that did not touch the assembled particles. Convective assembly (15] was used to order 500 nm polystyrene particles into 2D continuous layers. It was performed on flat PDMS templates previously oxidized by oxygen plasma. Capillary assembly [14] provided 500 nm polystyrene particle ass nblies with full control of the positions on patterned PDMS templates without any surface modification. PDMS templates were fabricated by injection molding on flat or patterned silicon masters [32]. They consisted of a 200 pm thick layer of custom-... 

Malaquin, L. et oL Controlled partide placement thiot h convective and capillary assembly. Langmuir (in the press). 

Figure 2. Partides selectively placed at designated traps on dedicated assembly templates by capillary assembly a) 200-nm particles in small traps, b) 350-nm particles in medium traps, c) 500-nm in large traps. Traps other than the designated ones remained empty. The assembly of 200-nm paiticle dimers in d) medium and e) large traps, and f) 350-nm particle dimers in large traps constitute assembly errors. The assembly direction is from right to left. Samples were o)ated with 10 nm gold for imaging enhancement using SEM. The scale bars are 2 pm.

Figure 3. a) Yields y and b) error rates (p of capillary assembly experiments with monomodal colloids plotted versus the trap type. Note the different scales of the y axes (a 100%, b 5%). 200-nm (P200), 350-nm (P350), and 500-nm particles (P500) were predominantly assembled in their corresponding trap types. 

Careful control of the flame temperature and heating time is critical to obtaining a well-sealed tip. Thus, the Pt tip/capillary assembly is held in the cone part of the flame near the nozzle outlet for 2 sec. The melted glass retracts from the tip on cooling, giving conical Pt electrodes with the base radius of the cone of 5-10 pm and the cone height of X10-20 pm, as shown in Figure 6.3.4.2. 

A schematic diagram of the high-shear capillary viscometer utilized in this study is presented in Figure 1. This apparatus is a modification of the version originally described by Fenske, Klaus and Dannenbrink (7), and details of the experimental apparatus and procedure have been presented by Lee (19). Also, details of the capillary assembly and the nitrogen pressure control system have been presented by Graham and co-workers (20). 

The capillary consists of a stainless-steel tubing with one end soldered into a fitting. Details of this capillary assembly are available (19,20). 

L. Malaquin, T. Kraus, H. Schmid, E. Delamarche, and H. Wolf, Controlled particle placement through convective and capillary assembly, Langmuir, 23,11513-11521 C2007). 

Using Capillary Forces for Directing Single-Particle Placement Capillary Assembly... 

The combined effects of capillary forces (resulting from the local distortion of the meniscus when the contact line is dragged over the structures) and geometrical confinement (induced by the structures) causes one or more particle being trapped close to the obstacle, whereas no particles are deposited in the surrounding areas (Fig. 15.Id). This capillary assembly mechanism is well suited for accurate placement of individual particles to create discontinuous ID structures (Fig. 15.3) and can be extended to 3D patterns. 

Figure 15.3 Examples of 500 nm PS particles (a and b) and 60-80 nm gold particles immobilized on a PDMS template using capillary assembly. " ...

The choice of the experimental setup is determinant and has to be done according to the complexity of the structure to be achieved. Many simple methods can provide straightforward routes for the manufacturing of extended 2D layers through convective assembly. However, the control of the precise positioning of objects in the capillary assembly mode is more restrictive and usually requires a more accurate control of the evaporation rate and/or control of the meniscus velocity. Systems that are capable of decoupling these two parameters are more flexible and offer versatile platforms assembly methods. The objective of this paragraph is to present an overview of the systems reported in literature and to help the reader to select the approach the more appropriate to its application with a balanced investment/performance ratio. 

Substrate dipping offers a better control of dewetting and seems to be well adapted to capillary assembly (Fig. 15.5a). It is usually derived from Langmuir-Blodgett methods. The template is usually placed vertically and dipped directly into the colloidal... 

Figure 15.5 Schematic illustrations of the experimental setups used for convective and capillary assembly [i] by withdrawing a template substrate immerged vertically in a colloidal suspension and (ii) by dragging a droplet of a colloidal suspension on a thermalized template using a translation stage.

However, contrary to capillary assembly (see the next section), the assembly of particles sedimenting quickly compared to the duration of the experiments (<10 min) or particles interacting strongly with surfaces is nearly impossible (results not shown). 

Whereas the behavior of the meniscus is different in both assembly mechanisms, evaporation plays a critical role in capillary assembly as well. Hydrodynamic flows created by evaporation tend to bring particles from the bulk suspension toward the contact line. In convective assembly, this accumulation of particles close to the contact line results directly in layer formation. In capillary assembly, even if capillary forces prevent deposition, it was recently shown that this process was essential to initiate particle deposition. - ... 

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