Michael J. Solomon

University of California, Berkeley
PhD ChE ’96

University of Wisconsin-Madison
BS ChE and Economics ’90

Our research investigates complex fluids and soft matter – soft materials with properties intermediate between fluids and solids. Our current interests include self-assembly of colloids, colloidal gelation, active matter, and the biomechanics of bacterial biofilms. Applications that interest us include creating new optical materials, sensors, biomedical devices and procedures, as well as materials for energy management. The potential to create new materials that reconfigure autonomously – and in response to environmental stimuli – is of particular interest to us.

Self-assembly
The assembly of colloids into useful structures has long been a key aim of chemical engineers and materials scientists. For example, ordered arrays of colloidal particles formed in the liquid state can be further processed to yield iridescent, structural color materials useful for sensing, optical materials, and control of the electromagnetic response of materials. There structures could be reconfigured to manipulate loads, as needed for microrobotic applications. Yet, the success of this technological aim is severely hindered by some deep fundamental problems. For example, the crystal structures that have been fabricated to date are disappointingly small, most likely because typical colloidal building blocks are not nearly complex as molecules. Structures are also quite defect prone. We address this challenge by synthesizing colloids—often anisotropic—and assembling them with the assistance of applied electric fields. In a second effort, we address the fact that the typical size of the ordered arrays that have been produced in academia is currently too small for real-world applications. These questions are studied within a collaborative, student-driven research program that includes novel colloid synthesis, direct visualization of assembly structure and dynamics by confocal microscopy, functional property characterization by light scattering and reflectivity measurements, and rheological measurements. This research is supported by DOE and involves collaboration with Professor Sharon Glotzer.

Colloidal gelation and active matter
Colloidal gelation is a common industrial process to manage the rheological and microstructural properties of complex fluid formulations used in the stabilization of consumer products, ceramic materials and pharmaceutical formulations. By developing new 3D confocal microscopy methods, our group has made fundamental discoveries about gels that are currently being applied in industry to develop new materials and complex fluid processing methods. Currently, we are engaged in investigation of the thixotropic response of gels in collaboration with Professor Ron Larson. A new direction of this work is to incorporate active matter into colloidal gels. By incorporating colloids with driven dynamics, we control and exploit the microrheological response of colloidal gels for technological applications. It is also a rich avenue for the scientific investigation of the interaction of active matter with passive colloids. This work involves a combination of advanced microscopy techniques, flow cell fabrication using methods such as microfabrication, and rheological measurements. This research is supported by NSF and Procter & Gamble.

Biomechanics of bacterial biofilms
With Dr. Scott vanEpps of the University of Michigan Department of Emergency Medicine, we are exploring the biomechanical properties of bacterial biofilms. Biofilms are colonies of microorganisms that are pervasive in a range of natural and industrial settings. They can also grow on devices, such as intravascular catheters, that are introduced into the body as part of medical practice. Biofilm structure and mechanics is thought to play a protective role by, for example, improving the resistance of bacteria to antibiotic treatments. The aim of this project is to understand and measure the mechanical properties of biofilms and understand physical variables that might be able to control and resolve their infection in medical devices. We are in particular engaged in exploiting heat effects on biofilm viability and structure as a control mechanism. We are also investigating the fundamental science of biofilm and host interactions. We also engaged in studies of the polymer science of the extracellular polysaccharides and other polysaccharide polymers, rheological characterization of whole biofilms, and confocal microscopy visualization of the complex microscopic structure of biofilms. This research is supported by NIH.

• Liu, T. and M.J. Solomon, “Reconfigurable Grating Diffraction Structural Color in Self-Assembled Colloidal Crystals,” Small 19 2301871 (2023). PMID: 37144433. DOI: 10.1002/smll.202301871
• Saud, K.T. and M.J. Solomon, “Microdynamics of active particles in defect-rich colloidal crystals,” J. Colloid Interface Science. 641 950-960 (2023). PMID: 36989821; DOI: 10.1016/j.jcis.2023.03.025   
• Saud, K.T., J. Xu, S. Wilkanowicz, Y. He, J.J. Moon, and M.J. Solomon, “Electrosprayed microparticles from inulin as poly(vinyl alcohol) for colon-targeted delivery of prebiotics,” Food Hydrocolloids 140 108625 (2023). DOI: https://doi.org/10.1016/j.foodhyd.2023.108625.
• Beckwith, J.K., M. Ganesan, J.S. VanEpps, A. Kumar, and M.J. Solomon, “Rheology of Candida albicans fungal biofilms,” Journal of Rheology 66(4) 683-697 (2022). DOI: 10.1122/8.0000427
• Liu, T., T. Liu, F. Gao, S.C. Glotzer, and M.J. Solomon, “Structural Color Spectral Response of Dense Structures of Discoidal Particles Generated by Evaporative Assembly,” J. Phys. Chem B 126 6 1315-1324(2022). DOI: 10.1021/acs.jpcb.1c10015
• Kao, P-K, M.J. Solomon, and M. Ganesan, “Microstructure and elasticity of dilute gels of colloidal discoids,” Soft Matter 18 1350-1363 (2022). DOI: 10.1039/D1SM01605A
• Liu, T., B. VanSaders, J.T. Keating, S.C. Glotzer, and M.J. Solomon, “Effect of Particles of Irregular Size on the Microstructure and Structural Color of Self-Assembled Colloidal Crystals,” Langmuir 37 13300 – 13308 (2021). https://doi.org/10.1021/acs.langmuir.1c01898
• Rocklin, D.Z., L.C. Hsiao, M. Szakasits, M.J. Solomon, and X. Mao, “Elasticity of colloidal gels: structural heterogeneity, floppy modes, and rigidity,” Soft Matter (2021). DOI: 10.1039/D0SM00053A
• Vitale, C., T.M. Ma, J. Sim, E. Martinez-Nieves, U. Kadiyala, M.J. Solomon, and J. Scott VanEpps, “Staphylococcus epidermidis has growth phase dependent affinity for fibrinogen and resulting fibrin clot elasticity,” Frontiers in Microbiology 12:649534 (2021). DOI: 10.3389/fmicb.2021.649534  
• Kao, P-K., B.J. VanSaders, S.C. Glotzer and M.J. Solomon, “Accelerated annealing of colloidal crystal monolayers by means of cyclically applied electric fields,” Scientific Reports 11 11042 (2021). DOI: 10.1038/s41598-021-90310-7
• Saud, K., M. Ganesan, and M.J. Solomon, “Yield stress behavior of colloidal gels with embedded active particles,” Journal of Rheology 65 225-239 (2021). DOI: 10.1122/8.0000163
• Beckwith, J.K., J.S. VanEpps, and M.J. Solomon, “Differential Effects of Heated Perfusate on Morphology, Viability, and Dissemination of Staphylococcus epidermidis Biofilms,” Applied and Environmental Microbiology 86:e01193-20 (2020). DOI:10.1128/AEM.01193-20. PMID: 32801173
• Liu, T., B. VanSaders, S.C. Glotzer, and M.J. Solomon, “Effect of Defective Microstructure and Film Thickness on the Reflective Structural Color of Self-Assembled Colloidal Crystals,” ACS Applied Materials and Interfaces 12 8 9842-9850 (2020). DOI: 10.1021/acsami.9b22913.

For a complete list of publications, please cf. Mike Solomon’s cv.