Surface Modifications for Flow Control

EOF can be suppressed by coating the channel surface with methylhydroxy-ethylcellulose (MHEC) [631], orphotopolymerized polyacrylamide [415]. Reversal of EOF was also achieved, e.g., the PMMA surface was modified with N-lithioethylenediamine or N-lithiodiaminopropane [1063] or the PDMS surface was coated by hydrophobic interaction with TBA+ [302]. 

To coat selectively only at some sections of a channel network, outflow of solvent at other sections should be used to prevent them from being wetted by the chemical modification reagents [148,766]. 

Polyelectrolyte multilayers (PEM) have been used to alter surface charges, thus controlling the direction of EOF in chips made of PS [155,216,416], PMMA [216], and PETG [416]. PEM deposition was carried out by exposing the micro-channel alternatively to solutions of positively charged PEM poly(allylamine hydrochloride) PAAH and negatively charged PEM-poly(styrene sulfonate), or 

FIGURE 3.14 Four flow patterns achieved in the intersection of the PS chip derivatized to have different surface charges on the various arms. The applied voltages are the same in all cases [216]. Reprinted with permission from the American Chemical Society. 

Surface modification can be achieved by 02 plasma oxidation. For instance, because of the increase in surface negative charges after oxidation, the oxidized PDMS surface supports EOF. However, because of the instability of the charge created on the polymer surface, EOF was unstable. Better stability can be achieved by immediately filling the PDMS channel with liquids, rather than letting it be exposed to air. The useful lifetime of these devices for quantitative CE analysis, which requires EOF stability, is probably 3 h [1033]. 

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