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Thesis Defense May 9th – Raja Maheedhara

Title: ENHANCED DIFFUSIOOSMOSIS AND THERMOOSMOSIS IN POLYELECTROLYTE-BRUSH-FUNCTIONALIZED NANOCHANNELS

Wednesday, May 9th at 10:00am

DeWalt Seminar Room (2164 Martin Hall)

Committee:

Dr. Siddhartha Das, Chair

Dr. Taylor Woehl

Dr. Amir Riaz

Abstract: In this thesis, flow augmentation principle of nanochannel grafted with end-charged PE brushes is utilized to investigate two different nanofluidic flow mechanisms. We fi rst study an ionic difusioosmotic (IDO) transport in such end-charged brush-grafted nanochannels. IDO refers to a flow mechanism triggered by the application an external axial concentration gradient: this gradient induces (a) an electric field that drives an EOS transport and (b) a pressure-gradient dictated by the different in the diffusivities of the ions triggering a chemioosmotic (COS) flow. The study carefully points out (a) this induced electric field, (b) the overall DO transport
and (c) the individual contributions of the EOS and COS flow fi elds in the overall DO transport. Our results unravel that the DOS transport is massively enhanced in nanochannels grafted with PE brushes with weak grafting density stemming from the significantly enhanced EOS transport caused by the localization of the EOS body force away from the nanochannel walls. This augmentation is even stronger for cases where the COS transport aids the EOS transport. On the other hand, the DOS transport gets severely reduced in nanochannels grafted with dense PE brushes owing to the severity of the brush-induced additional drag force.

Secondly, we study an ionic thermoosmotic transport (ITO) in such end-charged brush-grafted nanochannels. ITO refers to a flow mechanism triggered by the application an external axial temperature gradient. The issues that are important here include: (a) the induced electric field and (b) the overall TO transport dictated by the induced-electric- field EOS transport, COS transport caused by the diffusivity difference of the EDL ions and a thermophoretic (TPT) transport caused by the difference in the preferential transport of the ions following the imposed temperature gradient. Here too the overall ITO is signi ficantly enhanced as compared to the case of the brush-free nanochannels and we pinpoint the relative influence of the EOS, DOS, and the TPT transport in the overall ITO transport.