Microhole diameter

Figure 5.8d shows the radial temperature distribution at the drilling depth of 300 pm. The tool reaches a temperature of around 500°C, as measured experimentally by Kellog [70] and Basak and Ghosh [6,7]. The marked points are the microhole diameters. The temperature at the border of the microhole is the machining temperature. It is remarkable that for all simulated voltages, the machining temperature Tm is found to be similar (around 500-600°C). 

The theory has been verified by voltammetric measurements using different hole diameters and by electrochemical simulations [13,15]. The plot of the half-wave potential versus log[(4d/7rr)-I-1] yielded a straight line with a slope of 60 mV (Fig. 3), but the experimental points deviated from the theory for small radii. Equations (3) to (5) show that the half-wave potential depends on the hole radius, the film thickness, the interface position within the hole, and the diffusion coefficient values. When d is rather large or the diffusion coefficient in the organic phase is very low, steady-state diffusion in the organic phase cannot be achieved because of the linear diffusion field within the microcylinder [Fig. 2(c)]. Although no analytical solution has been reported for non-steady-state IT across the microhole, the simulations reported in Ref. 13 showed that the diffusion field is asymmetrical, and concentration profiles are similar to those in micropipettes (see... 

Luo et al. [61] demonstrated a simple method for the preparation of SERS active Ag nanostructures substrates by deposition of Ag nanoparticles into the designed Si holes. The morphologies of the Ag nanostructures were observed with SEM. The diameters of the Ag nanoparticles were found to be 40-60 nm. With increasing deposition time, flower-like Ag nanostructure commenced crystallization to form near the edge of the bottom surface of the Si microholes. These Ag nanostructures exhibited strong SERS enhancement, which provided an excellent platform for monitoring the R6G molecules by SERS technology [62]. 

The diameters of the microholes were chosen according to experimentally known over-cut (see Section 6.2.6). For simplification, it was further assumed that the lateral space between the tool-electrode and the glass is filled with the gas film. 

Figure 6.6 Mean entrance diameter of microholes obtained by SACE glass gravity-feed drilling as a function of the machining voltage and the drilling depth for a 0.4 mm stainless steel tool-cathode. Reprinted from [84] with the permission of the Journal of Micromechanics and Microengineering.

The promotion of the electrolyte flow results not only in reduced entrance diameters, but also in sharper microhole sidewalls and reduced roundness errors [136]. 

As discussed in Section 4.4.1, it is possible to reduce the critical voltage by changing the wettability of the electrode-electrolyte interface, which can be achieved by adding surfactants to the electrolyte. An example is shown in Fig. 7.9(a). Liquid soap was added to 30 wt% NaOH [129]. The critical voltage is reduced from around 30 to about 14 V. The critical current density and the gas film formation time are also reduced. Machining at lower voltages becomes possible. An example of successive drillings of microholes at 20 V is illustrated in Fig. 7.9(b). Very well-defined contours are achieved. The fluctuation of the mean diameter is less than 5 Xm (computed from a set of 50 microholes), which... 

The entrance diameter decreases with T due to the reduced mean heat power supplied to the workpiece. Another reason for this behaviour is the periodic destruction of the gas film. This allows the electrolyte to flow inside the microhole and therefore results in a more homogenous chemical etching of the hole compared with DC voltage machining where the spark activity progressively moves to the top of the tool-electrode as the drilling goes deeper. 

Microholes, slots, and channels can be fabricated successfully by EMM with high precision and quality. Figure 9.3 shows the microhole of entry diameter 24 pm and exit diameter 22.5 pm drilled on an SS-304 plate. EMM operation was performed at 5 MHz pulse power, pulse width 60 ns, average voltage 2.8 V, amplitude of vibration of microtool 0.2 pm, and frequency of vibration of microtool 236 Hz. 

Rubinsky s group presented the first microfluidic device to electroporate a cell (Davalos et al., 2000 Huang and Rubinsky, 1999). Their devices consisted of three silicon chips bonded together to form two chambers, separated by a 1 pm thick silicon nitride membrane with a 2—10 pm diameter hole. Because silicon nitride is nonconductive, any electrical current flowing from the top chamber to the bottom chamber must pass through this microhole. A cell suspension was introduced into the top chamber, followed by the immobilization of one cell in the hole by lowering the pressure in the bottom chamber. 

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