Feb. 23: "Characterizing phase transitions of microfibrillated cellulose induced by anionic and cationic surfactants"
Shiqin He, graduate student in the Department of Chemical and Biomolecular Engineering (ChBE) at Lehigh University was chosen to present her rheology research findings as part of the Spring 2023 Society of Rheology (SOR): Future of Rheology Seminar Series
The selection is through the self nomination to the Society of Rheology and decided by the organizing committee. Each year will have two rounds of abstract solicitations. 
He is from Wuhan, China and holds a BS in Chemical Engineering from the University of Iowa, 2016. Following her BS degree, He held a research assistant internship at Humanwell Healthcare (Group) Co., Ltd, where she collaborated with the R&D team to derive and improve pharmaceutical excipients process procedure to achieve higher purity. He is pursuing her PhD in Colloid, Rheology, Sol-gel Transition from Lehigh University, 2022, and has continued on with an anticipated May 2023 graduation date. 
He is advised by ChBE Professor Kelly Schultz. Her research focus is the study of rheology on colloidal materials and her first project was directed on the rheological properties of phase transitions in polydisperse and monodisperse colloidal rod systems. This work ultimately led to a publication in the AIChE Journal in 2021, which was recognized as an Editor's Choice paper for the November issue. Following the completion of her PhD, Shiqin continued to conduct research on rheology of polydisperse and monodisperse colloidal rod systems. She authored a publication in RSC Advances in 2022, which focused on measuring the gelation phase diagrams of colloidal rod systems over a large composition space. This publication is part of the RSC Advances Emerging Investigator Series. Her work now is focused on the characterization of rheological phase transition on microfibrillated cellulose.
Rheological modifiers are often used to tune rheology or induce phase transitions of products. Fibrous colloids are common rheological modifiers because they can tune rheology with the addition of a small amount of material due to their aspect ratio. Hydrogenated castor oil (HCO) is a colloidal fibrous rheological modifier used in fabric and home care products. The anionic surface of HCO limits the use of this colloid because it is only compatible with anionic surfactant formulations. Microfibrillated cellulose (MFC), another fibrous colloid with similar aspect ratios to HCO, has a non-ionic surface which is compatible with more formulations including anionic, cationic and non-ionic surfactants. In addition, MFC is a paper industry waste product, making it a renewable and abundant material. These properties make MFC a potentially effective substitute for HCO and could also broaden its use in diverse formulations. However, the addition of MFC to products as a rheological modifier has been slow because of a lack of rheological studies on the changes in material properties and structure during phase transitions. This work modifies the surface of MFC and characterizes the rheological properties and structure of MFC when phase transitions are induced by contact with either anionic or cationic surfactants. A carboxyl group is first introduced to the surface of MFC using TEMPO oxidation. Then the oxidized MFC (OMFC) is contacted with an anionic surfactant, sodium dodecyl sulfate (SDS), or a cationic surfactant, benzyldimethyldodecylammonium bromide (BDDAB), to induce gelation. This gelation process is characterized using multiple particle tracking microrheology (MPT).
MPT tracks the Brownian motion of fluorescence probe particles embedded in a sample, which is related to the sample rheological properties. From the MPT results, we determine that the rheology and microstructure of OMFC during gelation in anionic and cationic surfactant solutions are different. Gelation of OMFC in anionic surfactant is gradual and the OMFC sample shrinks during network formation. In contrast, gelation of OMFC in cationic surfactant is rapid. The OMFC sample gels almost instantaneously after it is contacted with the cationic surfactant. This gel remains in its original shape without any shrinkage. These two distinct gelation processes indicate that the OMFC gelation mechanism depends on the charge of the contacting surfactant. When OMFC is gelled in an anionic surfactant, OMFC colloidal fibers are able to rearrange into a more compact structure. However, when OMFC is gelled in a cationic surfactant, fast gelation limits the ability for the fibers to rearrange. This allows the material to retain its original shape without any shrinkage. The data collected in this work shows how the rheology of a product may evolve depending on the contacting surfactant charge. This can be used to guide future design of products where rheological properties are specified from existing data considering the contacting surfactant that induces phase transitions.