2026 Student Poster Award - 1st Place

Quantifying Phase Separation and Local Transport in Thermoresponsive PNIPAm Solutions Using Multiple Particle Tracking Microrheology

Nazrin Hasanova | Lehigh University
Maria Chiara Roffin | University of Leicester
Xuanhong Cheng | Lehigh University
Kelly Schultz, PhD | Purdue University
James Gilchrist, PhD | Lehigh University

Abstract

Thermoresponsive polymers, such as poly(N-isopropylacrylamide) (PNIPAm), have potential use in temperature-controlled particle separation. Above the lower critical solution temperature (LCST) of ~34°C, PNIPAm chains collapse, expel water and form polymer-rich domains that confine particle motion. While the bulk phase behavior of PNIPAm is well characterized, the local rheology during phase separation is poorly understood. To characterize this, we apply a one-dimensional thermal gradient in the x-direction to generate a phase-separated interface in PNIPAm solutions of increasing polymer concentration in the semi-dilute regime. To quantify local material properties across the interfacial region, we track suspended probe particles using 2D multiple particle tracking microrheology. We compare experiments in ground and microgravity conditions to measure the effect of gravity on PNIPAm phase separation. In microgravity, phase separation results in localization of particles at the hot side. In ground conditions, the thermal gradient induces buoyancy-driven recirculation in the x-direction, driving continuous flow of polymer solution toward the interface. Motion in the y-direction is diffusive and characterizes local material properties. van Hove correlation functions of particle displacements in each direction identify two distinct particle populations with different mobilities. These populations correspond to particles in different regions of the phase-separated system, with slow particles in the polymer-rich phase and fast particles in the solvent-rich phase. The spatial distributions of populations across the interface, apparent particle diffusivity, local heterogeneity and particle velocity measured in each population are independent of PNIPAm concentration. This indicates that phase separation results in a reproducible interfacial region with consistent microscale properties. This work informs the design of controlled thermal-gradient driven separation processes.

1^st Place, 97^th Annual Meeting, Oct 2026, Boston