title: Combinational concentration gradient confinement through stagnation flow.
abstract: Concentration gradient generation in microfluidics is typically constrained by two conflicting mass transport requirements: short characteristic times (τ) for precise temporal control of concentration gradients but at the expense of high flow rates and hence, high flow shear stresses (σ). To decouple the limitations from these parameters, here we propose the use of stagnation flows to confine concentration gradients within large velocity gradients that surround the stagnation point. We developed a modified cross-slot (MCS) device capable of feeding binary and combinational concentration sources in stagnation flows. We show that across the velocity well, source-sink pairs can form permanent concentration gradients. As source-sink concentration pairs are continuously supplied to the MCS, a permanently stable concentration gradient can be generated. Tuning the flow rates directly controls the velocity gradients, and hence the stagnation point location, allowing the confined concentration gradient to be focused. In addition, the flow rate ratio within the MCS rapidly controls (τ ∼ 50 ms) the location of the stagnation point and the confined combinational concentration gradients at low flow shear (0.2 Pa < σ < 2.9 Pa). The MCS device described in this study establishes the method for using stagnation flows to rapidly generate and position low shear combinational concentration gradients for shear sensitive biological assays.
authors.affiliation: School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore. mcyang@ntu.edu.sg and Microfluidics Manufacturing Programme, Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, 63807 Singapore.
authors.forename: Toh G G
authors.initials: TG
authors.lastname: Alicia
authors.affiliation: School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore. mcyang@ntu.edu.sg.
authors.forename: Chun
authors.initials: C
authors.lastname: Yang
authors.affiliation: Microfluidics Manufacturing Programme, Singapore Institute of Manufacturing Technology, 71 Nanyang Drive, 63807 Singapore.
authors.forename: Zhiping
authors.initials: Z
authors.lastname: Wang
authors.affiliation: QLD Micro- and Nanotechnology Centre, Nathan campus, Griffith University, 170 Kessels Road QLD 4111, Australia. nam-trung.nguyen@griffith.edu.au.
authors.forename: Nam-Trung
authors.initials: NT
authors.lastname: Nguyen