Staged phase separation in I–I–N tri-phase region of platelet–sphere mixtures

Significance 

It is important to understand the equilibrium states of mixtures and how colloidal mixtures with varying particle shapes and sizes attain equilibrium. This is stemming from the fact that these mixtures are naturally available and have been widely implemented in industrial applications.

Several researchers have studied previously the kinetics of phase separation using experiments, theory, and simulation. Unfortunately, studying phase transition kinetics is limited by the inherent theoretical challenges in characterizing the phase ordering process and obtaining an adequate experimental system.

The experimental incorporation of a second component in a colloidal suspension has indicated a number of new phenomena, which include dependence of demixing on size ratio, phase transitions, and reentrant phase boundary caused by the depletion interaction. A good number of colloidal mixtures tend to gel, therefore, this presents a difficulty in studying phase transitions in mixtures. Also, the high polydispersity of colloidal particles affects significantly phase transitions.

Researchers at Guangdong University of Technology in China, Mingfeng Chen, Min He, Pengcheng Lin, and Professor Ying Chen in collaboration with Professor Zhengdong Cheng from Texas A&M University presented a staged phase transition study where several pathways exist to realize a tri-phase equilibrium. They used highly anisotropic zirconium phosphate platelets to make an I-N transition at a low volume fraction due to their small aspect ratio. Their research work is published in peer-reviewed journal, Soft matter.

The authors prepared a series of samples with volume fractions of Zirconium Phosphate nanolayers ranging from 0.0054 to 0.0150 while the silica sphere ranged from 0 to 0.0041. In the phase transition processes, the authors found the samples first separated into two metastable phases, which would further develop to attain a tri-phase coexistence. The authors categorized the pathways of the samples into three main categories by implementing the method that was used for colloidal-polymer mixtures.

Experimental results indicated that experimental phase fractions were consistent with the calculated phase fractions qualitatively. However, deviations were expected mainly because of gravitational compression, while the error in computation of the experimental phase fractions and the computation due to the error in the proposed sample positions in the tri-phase coexistence also played a role.

The research team observed staged phase transitions inside the tri-phase triangle in the sphere-plate mixtures. These observations indicated that the initial homogenized specimens originally formed one or two metastable phases. Subsequently, these metastable phases separated into stable phases. The samples finally attained, gas, liquid, and nematic phase coexistence. Even though the specimens inside the tri-phase triangle finally attained equilibrium among the same gas, liquid, and nematic phases, these specimens exhibited distinct pathways.

The authors illustrated this feature by correlating with the free-energy landscape, extending the already developed method for sphere-polymer mixtures. More pathways awaits to be explored experimentally in the proposed platelet-sphere mixtures according to the pathway categories that has been discussed for tri-phase transitions.

Staged phase separation in the I–I–N tri-phase region of platelet–sphere mixturese. Advances in Engineering

Schematic of liquid crystal droplet with a protective gas layer, which is formed in the first step of the phase transition when the nematic phase can’t coexist with the liquid phase.

About the author

Mingfeng Chen, a Ph.D student in Guangdong University of Technology, Guangzhou, China. Since 2013, His research direction is the synthesis of platelet materials (α-Zirconium phosphate and kaolinite) and the self-assembly of discotic liquid crystals.

He mainly focuses on the effect of additives (spheres, salt, and so on) on the phase transition of the monolayers platelets (ZrP). Using ZrP as a platelet model is mainly due to the features of controllable of diameter and polydispersity, high diameter to thickness ratio, and non-formation of gel with addition of extra particles, which all are play positive roles in the phase transition investigations.

About the author

Ying Chen is a professor of Guangdong University of Technology, Guangzhou, China. She obtained her Ph.D in Southern China University of Technology in 2003 and has engaged in the research work for more than 20 years.

Her main research directions are heat transfer in micro/nano interface heat transfer, micro/nano fluid, preparation and characterization of microcapsule, the enhancement of condensation heat transfer and energy saving technology of air conditioning unit and heat pump. She has presided for more than 20 projects, which includes Key/Face foundation of National Natural Science of China, the provincial major projects, and so on.

About the author

Zhengdong Cheng is currently a professor at Artie McFerrin Department of Chemical Engineering, Texas A&M University, USA. He obtained his Ph.D in Physics from Princeton University in 1999, and was a postdoc at Princeton University, Exxon Mobile research & engineering Co., and Harvard University from 1999 to 2004. His research interest focuses on Soft Matter. Since 2004, Dr. Cheng has been working on active soft matter based on nonlinear chemical reactions. His group is the first in the US conducting experimental research on self-oscillating hydrogel particles, where Belousov-Zhabotinsky reaction catalysts were incorporated in the polymeric network functioning as the engine that transformed chemical energy to mechanical energy to power the automotive motion of the polymer.

Dr. Cheng developed a model system to investigate communication among BZ oscillators. As the second research direction, with funding from ACS Petroleum Research, NSF and NASA, Dr. Cheng has been conducting research on discotic liquid crystals. Achievements include the discovery of discotic smectic phase, surface controlled shape design, gelation via ion exchange, and mass fabrication of Janus platelets for nanoencapsulation. Strong industrial support came from Chevron, Wintershall of BASF, Dong Energy, as well as China Petroleum. The third research direction is hydrogen production via water splitting using sun light. Dr. Cheng is also very passionate about Microfluidics. He pioneered, following Prof. Dave Weitz since 2002, in electric control of droplet microfluidics. He devotes himself to the manufactory of microfluidic devices and systems, serving the biomedical industry in Gene sequencing and POCT, Agricultures, Petroleum, as well as food and water safety.

Reference

Mingfeng Chen, Min He, Pengcheng Lin, Ying Chena and Zhengdong Cheng. Staged phase separation in the I–I–N tri-phase region of platelet–sphere mixtures. Soft Matter, issue 13 (2017), pages 4457—4463.

 

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