Microscale investigation on the wettability and bonding mechanism of oxygen plasma-treated PDMS microfluidic chip

The bonding of polydimethylsiloxane (PDMS) microfluidic chip is one of the major concerns during the fabrication process. Plasma treatment can be applied to produce the hydrophilic groups, thus forming an irreversible bonding permanently. The surface wettability and bonding mechanism at the microscale level after plasma treatment need further investigation. In this study, the contact angle, surface morphology and the chemical composition of PDMS surface that were modified by oxygen plasma treatment were experimentally analyzed. Accordingly, molecular dynamics simulation models were constructed to investigate the wettability and bonding mechanism. Experimental results show that the contact angle decreases rapidly with the treatment time. The change in wettability mainly comes from the exposure of hydrophilic -OH and -CH2OH groups on silicon atoms, while the slight change in surface morphology can be negligible. With the help of hydroxyl groups, the spreading process of water droplet on the PDMS surface becomes much quicker because strong intermolecular interactions are generated. During the bonding process, atoms in both layers are gradually pushed towards the interface and penetrate each other, thus forming a reliable bonding at the interface. It is revealed in the debonding simulation that the plasma treatment on PDMS surface improves the bonding strength.