Khaled Younes

All Publications

• A self-consistent analysis of cluster morphology in supercritical carbon dioxide from Small Angle X-ray Scattering CHEMICAL PHYSICS LETTERS Muhunthan, P., Majumdar, A., Younes, K., Vignat, G., Li, H., Rajkovic, I., Weiss, T., Sokaras, D., Ihme, M. 2025; 876

  View details for DOI 10.1016/j.cplett.2025.142190

  View details for Web of Science ID 001513620200002

• Supercritical Ethanol-CO2 Mixtures Exhibit Microscopic Immiscibility: A Combined Study Using X-ray Scattering and Molecular Dynamics Simulations. The journal of physical chemistry letters Fan, J., Yoon, T., Vignat, G., Li, H., Younes, K., Majumdar, A., Muhunthan, P., Sokaras, D., Weiss, T., Rajkovic, I., Ihme, M. 2025: 7090-7099

  Abstract

  Supercritical mixtures of ethanol (EtOH) and carbon dioxide (CO2) are classified as type-I mixtures, with complete macroscopic miscibility. However, differences in molecular polarity and interactions suggest a distinct phase behavior at the microscopic level. Here, we combine small angle X-ray scattering experiments and molecular dynamics (MD) simulations to investigate the microscopic structure of EtOH-CO2 mixtures under supercritical conditions. The structure factor exhibits nonlinear composition-dependent behavior, revealing pronounced local density fluctuations. The complementary MD simulations, using optimized force field parameters, provide atomistic insight, showing that EtOH forms self-associated, hydrogen-bonded aggregates, while CO2 remains more uniformly distributed. Cluster analysis identifies a preferential EtOH-rich composition exceeding the bulk average, governed by a balance between energetic and entropic competition. These results demonstrate that, contrary to macroscopic expectations, the mixture exhibits significant microscopic heterogeneity and immiscibility, which may influence solubility, reactivity, transport properties, and thermodynamic response functions. These findings challenge the conventional views of type-I fluids and emphasize the necessity of revising mixture states and considering molecular polarity.

  View details for DOI 10.1021/acs.jpclett.5c01293

  View details for PubMedID 40604336

• A versatile pressure-cell design for studying ultrafast molecular-dynamics in supercritical fluids using coherent multi-pulse x-ray scattering. The Review of scientific instruments Muhunthan, P., Li, H., Vignat, G., Toro, E. R., Younes, K., Sun, Y., Sokaras, D., Weiss, T., Rajkovic, I., Osaka, T., Inoue, I., Song, S., Sato, T., Zhu, D., Fulton, J. L., Ihme, M. 2024; 95 (1)

  Abstract

  Supercritical fluids (SCFs) can be found in a variety of environmental and industrial processes. They exhibit an anomalous thermodynamic behavior, which originates from their fluctuating heterogeneous micro-structure. Characterizing the dynamics of these fluids at high temperature and high pressure with nanometer spatial and picosecond temporal resolution has been very challenging. The advent of hard x-ray free electron lasers has enabled the development of novel multi-pulse ultrafast x-ray scattering techniques, such as x-ray photon correlation spectroscopy (XPCS) and x-ray pump x-ray probe (XPXP). These techniques offer new opportunities for resolving the ultrafast microscopic behavior in SCFs at unprecedented spatiotemporal resolution, unraveling the dynamics of their micro-structure. However, harnessing these capabilities requires a bespoke high-pressure and high-temperature sample system that is optimized to maximize signal intensity and address instrument-specific challenges, such as drift in beamline components, x-ray scattering background, and multi-x-ray-beam overlap. We present a pressure cell compatible with a wide range of SCFs with built-in optical access for XPCS and XPXP and discuss critical aspects of the pressure cell design, with a particular focus on the design optimization for XPCS.

  View details for DOI 10.1063/5.0158497

  View details for PubMedID 38170817

• Autonomous screening of complex phase spaces using Bayesian optimization for SAXS measurements NUCLEAR INSTRUMENTS & METHODS IN PHYSICS RESEARCH SECTION A-ACCELERATORS SPECTROMETERS DETECTORS AND ASSOCIATED EQUIPMENT Younes, K., Poli, M., Muhunthan, P., Rajkovic, I., Ermon, S., Weiss, T. M., Ihme, M. 2023; 1057

  View details for DOI 10.1016/j.nima.2023.168719

  View details for Web of Science ID 001088640900001

• Autonomous screening of complex phase spaces using Bayesian optimization for SAXS measurements. Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment Younes, K., Poli, M., Muhunthan, P., Rajkovic, I., Ermon, S., Weiss, T. M., Ihme, M. 2023; 1057

  Abstract

  The advent of modern, ultrafast X-ray experiments has enabled scientists to probe physical phenomena at an ever smaller scale. However, this has come at a cost of excessive data generation, to the point where current storage and hardware capabilities are routinely surpassed. As such, handling the data efficiently and selectively storing only the information of most relevance is crucial. In this paper, we propose to use Bayesian optimization as a method to alleviate this problem. We apply the method to locate global features in Small Angle X-ray Scattering spectra obtained from conducting experiments with supercritical CO2. By evaluating the algorithm on more than 250 data points, we show that the implementation is versatile, robust, and computationally efficient, often converging with just a few iterations and with a minimal error penalty. This paves the way for creating fully autonomous experiments, where data science algorithms such as the one presented herein operate hand-in-hand with the expert user to maximize scientific discovery and minimize the associated experimental cost.

  View details for DOI 10.1016/j.nima.2023.168719

  View details for PubMedID 40880750

  View details for PubMedCentralID PMC12382457

• Improving volume-averaged simulations of matrix-stabilized combustion through direct X-ray μCT characterization: Application to NH₃/H₂-air combustion COMBUSTION AND FLAME Zirwes, T., Vignat, G., Toro, E. R., Boigne, E., Younes, K., Trimis, D., Ihme, M. 2023; 257

  View details for DOI 10.1016/j.combustflame.2023.113020

  View details for Web of Science ID 001077127800001

• Experimental and numerical investigation of flame stabilization and pollutant formation in matrix stabilized ammonia-hydrogen combustion COMBUSTION AND FLAME Vignat, G., Zirwes, T., Toro, E. R., Younes, K., Boigne, E., Muhunthan, P., Simitz, L., Trimis, D., Ihme, M. 2023; 250

  View details for DOI 10.1016/j.combustflame.2023.112642

  View details for Web of Science ID 000945070400001

• Mean Velocity Scaling of High-Speed Turbulent Flows Under Nonadiabatic Wall Conditions AIAA JOURNAL Younes, K., Hickey, J. 2022

  View details for DOI 10.2514/1.J062547

  View details for Web of Science ID 000892666000001

• A fuzzy cluster method for turbulent/non-turbulent interface detection EXPERIMENTS IN FLUIDS Younes, K., Gibeau, B., Ghaemi, S., Hickey, J. 2021; 62 (4)

  View details for DOI 10.1007/s00348-021-03169-9

  View details for Web of Science ID 000631035600004