[Paper Review] Soft-Pinning: Experimental Validation of Static Correlations in Supercooled Molecular Glass-forming Liquids

Rajsekhar Das, Bhanu Prasad Bhowmik+3 more|arXiv (Cornell University)|Jun 11, 2021

Material Dynamics and Properties4 citations

TL;DR

This study introduces and experimentally validates 'soft-pinning'—using dilute, slow-diffusing sorbitol molecules as pinning sites in supercooled glycerol to probe static correlation length scales. Dielectric spectroscopy reveals a concentration-dependent dielectric storage modulus, enabling direct measurement of static length scales via pinning susceptibility, confirming the method's robustness in molecular glass-formers and opening new pathways for experimental study of amorphous order near the glass transition.

ABSTRACT

Enormous enhancement in the viscosity of a liquid near its glass transition is generally connected to the growing many-body static correlations near the transition, often coined as `amorphous ordering'. Estimating the length scales of such correlations in different glass-forming liquids is highly important to unravel the physics of glass formation. Experiments on molecular glass-forming liquids become pivotal in this scenario as the viscosity grows several folds ($\sim 10^{14}$), simulations or colloidal glass experiments fail to access the long-time scales required. Here we design an experiment to extract the static length scales in molecular liquids using dilute amounts of another large molecule as a pinning site. Results from dielectric relaxation experiments on supercooled glycerol with different pinning concentrations of sorbitol and the simulations on a few model glass-forming liquids with pinning sites indicate the robustness of the proposed method, opening a plethora of opportunity to study the physics of other glass-forming liquids.

Motivation & Objective

• To experimentally measure static correlation length scales in molecular glass-forming liquids, which remain challenging to probe due to extreme viscosity and long relaxation times.
• To validate the theoretical concept of 'soft-pinning'—using slow-diffusing impurities as effective pinning sites—under experimentally accessible conditions.
• To establish a robust, experimentally feasible method for quantifying static length scales in supercooled liquids using dielectric response to pinning agents.
• To bridge the gap between simulation-based predictions and experimental observation of growing static correlations near the glass transition.

Proposed method

• The experiment uses ultra-pure glycerol as the glass-forming liquid and sorbitol as a soft pinning agent due to its significantly lower diffusion coefficient compared to glycerol.
• Dielectric relaxation measurements are performed using a home-built parallel-plate capacitor setup with stainless steel electrodes and Teflon insulation, thermally isolated in a Dewar flask with liquid nitrogen cooling.
• The dielectric storage modulus ε′(T, c, t) is measured across temperatures from 203 K to 248 K and sorbitol concentrations from 0% to 15%.
• Pinning susceptibility χₚ^Expt(T, t) is calculated as the derivative of ε′ with respect to concentration c at c = 0, using the expression χₚ^Expt = [ε′(c, T, t) − ε′(c=0, T, t)] / c at c = 0.
• A modified Havriliak-Negami function is used to rescale ε′ to 0–1 for normalization, but no fitting is applied to the core data analysis.
• Simulations on model glass-forming liquids with pinning sites are performed to validate the method’s consistency and robustness across systems.

Experimental results

Research questions

• RQ1 Can soft-pinning with dilute, slow-diffusing solutes be used to experimentally probe static correlation length scales in molecular glass-forming liquids?
• RQ2 Does the dielectric response of a supercooled liquid exhibit measurable changes in response to soft pinning, enabling extraction of pinning susceptibility?
• RQ3 How does the pinning susceptibility scale with temperature and concentration, and can it be used to extract the static correlation length?
• RQ4 Is the soft-pinning method robust and transferable across different glass-forming systems, as validated by simulations?
• RQ5 Can this method provide experimental access to static correlations in molecular liquids where traditional methods fail due to long relaxation times?

Key findings

• The pinning susceptibility χₚ^Expt(T, t) extracted from dielectric measurements shows a clear temperature and time dependence, confirming the method’s sensitivity to dynamic and static correlations.
• The measured dielectric storage modulus ε′(T, c, t) exhibits a concentration-dependent shift at c = 0, enabling the calculation of χₚ^Expt(T, t) with high precision.
• The static correlation length ξ increases with decreasing temperature, consistent with theoretical expectations of growing amorphous order near the glass transition.
• Simulations on model systems confirm that the soft-pinning method reliably extracts static correlation lengths, validating its robustness across different liquid models.
• The method successfully accesses length scales in molecular liquids where conventional experiments fail due to viscosity-induced long relaxation times.
• The experimental setup achieves stable temperature control for 3–4 minutes per window, enabling reliable dielectric measurements across the supercooled regime.