www.acsprf.org

We aim to understand thermophoresis, or mass migration due to a temperature gradient.  This effect has been used to separate molecules of different sizes and composition and has great promise to be used in microfluidic devices for chemical analysis.  However, the underlying mechanism is elusive since many factors can affect thermophoresis. 

We synthesize a dye-labeled  N-isopropylacrylamide (NIPAM) and N-acryloxysuccinimide (NASI) copolymer.  When in aqueous solution, the polymer undergoes a transition from the coil to globule state at the cloud point temperature.  The cloud point is sensitive to the amount of NaCl co-dissolved in solution with the polymer.  If subjected to a temperature gradient with the average temperature equal to the cloud point, both forms of the polymer are present.

We study how the coil-globule transition, average temperature and salt concentration affect thermophoresis, or mass migration due to a temperature gradient, in both simulation and experiment.  The effect can be characterized using the Soret coefficient, S_T.  For a uniform temperature gradient, dt/dy, the concentration gradient will be exponential, c(y)=c₀*exp(S_T*dt/dy*y).  Here y rpresents position and c₀ is the uniform concentration of the polymer when no temperature gradient is applied.

The experiments use circulating hot and cold water to produce a temperature gradient across a sample channel of thickness 1cm.  A digital picture of the sample is analyzed to determine the concentration of the polymer in solution.  We have observed that the globule conformation has a negative Soret coefficient and migrates to the temperature maximum, while the coil form migrates to the temperature minimum.  This leads to a concentration minimum in the center of the channel at the cloud point temperature. The Soret coefficient for both conformations can be computed from the concentration profile.  We have found that both forms are fit with the same theoretical prediction for the temperature dependence of the Soret coefficient.  We have also tested different concentrations of co-dissolved NaCl.  Increasing the salt concentration decreases the cloud point.  We have observed that this also increases the Soret coefficient for the same average temperature.

We use a lattice-Boltzmann-based simulation with a bead-spring polymer model to investigate the mechanism behind the sign change of the Soret coefficient.  We have implemented a Lennard-Jones potential to model the interactions of the polymer with itself and the surrounding fluid.  This potential has been used to model the coil to globule transition for other polymers.  The transition is noted when the magnitude of the interaction is varied with respect to the magnitude of the thermal energy.  We implement a potential that varies linearly with position to mimic the conditions of experiment.  We find that changing the sign of the slope of the potential’s dependence on position determines if the polymer migrates to the hot or the cold side of the channel.

 We are currently investigating how ST varies quantitatively while varying the magnitude of the slope of the potential.  In experiment, we are also investigating how system size affects the migration of the polymer.  We are currently designing and building smaller channels in which to do experiments.  We hope to understand the