Question

Despite hydrophobic recovery shortcomings, surface activation methods are widely used for rendering PDMS hydrophilic. Why?

Answer 1

The PDMS is widely used in microfluidics applications because it is a polymer which is liquid at room temperature because its glass transition temperature lies below the room temperature.But the question arises why we go for making the surface of PDMS hydrophillic while it doesn't remain permanently hydrophillic.So the answer to this question is to do the fundamental studies like flow dynamics in a microchannel where because of the hydrophobic behaviour of PDMS liquid will not want to wet the surface and hence there will be a large velocity gradient at the wall which will result in high viscous dissipation and wall shear.Now,if we modify the nature of PDMS by doing some plasma treatment which makes the surface hydrophillic,liquid will now adhere to the wall because of hydrophillic behaviour of the surface and hence,there will be slip at the surface.The resultant of which we will have low velocity gradient at the wall which will caus low shear stress and viscous dissipation.So the conclusion is that in order to minimise the resistance offered by the PDMS wall in microchannel we go for surface treatment.One important characteristic you might have observed for the flow through a pipe that if the roughness of the surface increases ,then friction factor also increases which provides a larger resistance to the flow.But in case of PDMS microchannel ,when the surface was done rough,actually the flow through microchannel was much faster as compared to hydrophillic PDMS behaviour because of Superhydrophobic behaviour of the wall.How this was done ?So with increase in the roughness of the pipe,the air molecules remain intact at the surface which leads to formation of Cassie Baxter state of wetting which provides the contact of the liquid with air and hence the resultant wall shear is much reduced because the liquid is now in contact with air which offers less resistance because of much less viscosity. (I can tell you the paper if you are interested)

Order of Wall resistance in microchannel: Hydrophobic > Hydrophillic > Superhydrophobic

But one question again arises in the mind that if the hydrophillic behaviour of PDMS is not permanently retained,then why shouldn't we opt for other material which are hydrophillic ?So the answer is yes definitely we can go for other material as well like Silicon wafer,copper,etc. for making a microchannel but we didn't go because they are very costly and silicon wafer is much fragile while for copper it will be difficult to fabricate.So the PDMS is used for all practical purposes because it is cost effective and widely available.