A microfluidic-based In-situ investigation of swelling dynamics of superabsorbent polymer microparticles

This paper presents a novel microfluidic-based technique for in-situ time-lapse investigation of the dynamic water absorption (or swelling) of individual superabsorbent polymer (SAP) microparticles (MPs). Most prior studies were restricted to using gravimetric techniques to investigate the equilibrium state of the swelled SAP macroparticles, without considering their swelling dynamics. Time-lapse investigation of the swelling rate is essential for optimizing SAP-MPs' performance to meet the needs of different applications. Moreover, since swelling occurs through the surface of SAP particles, we hypothesize that decreasing their size to microscale (i.e., SAP-MPs) could potentially improve their performance due to the increased surface area-to-volume ratio. In this context, the proposed microfluidic device enabled hydrodynamic trapping of an array of individual SAP-MPs and in-situ investigation of their swelling behaviour under a microscope. The SAP-MPs were initially suspended in ethanol and loaded into the microfluidic device. Once they were trapped, the fluid medium was switched to water to trigger their swelling. Acrylic acid (AA)/acrylamide (Aam)-based SAP-MPs in size range of 250-300 mu m were investigated to characterize the device's performance. The SAP-MPs' average trapping time and yield in the device were 9.56 s per SAP-MP and 92%, respectively, at an ethanol loading flow rate of 0.15 mL/min. A minimum water flow rate of 5 mL/min was required to achieve an average ethanol-to-water transition time of 10 s. This was needed to suppress the effect of the solution exchange on the swelling behavior of the SAP-MPs. The equilibrium volumetric swelling ratio (VSReq), swelling rate (SR), and initial SR of AA-Aam-based SAP-MPs were determined to be 5.59 +/- 0.13 (m(3)/m(3)), 0.039 +/- 0.002 (m(3)/m(3).s), and 0.068 +/- 0.010 (m(3)/m(3).s), respectively. A significant effect of the neutralization degree (ND) on the swelling behavior of AA-Aam-based SAP-MPs was also observed. At 90% ND, the maximum VSReq of 79.45 +/- 2.49 m(3)/m(3) and SR of 1.11 +/- 0.047 m(3)/m(3).s were achieved. This microfluidic platform could be used to conduct parametric study and real-time characterization of various SAP-MPs in biological, chemical, medical, and agricultural applications.