Physics of Complex Fluids Group (PCF)

The research goal of PCF is to understand and to control liquids and their interfaces from molecular to macroscopic scales. Our research connects fundamental phenomena in static and dynamic wetting, nanofluidics, microfluidic two-phase flow, functional surfaces, drop impact, and drop evaporation to practically relevant applications such as enhanced oil recovery, lab-on-a-chip systems, analytical chemistry (MALDI-MS), optofluidics, and inkjet printing.

Research areas

Application areas


Negative Emission Technologies (NETs) are a key requirement in most climate models that restrict global warming to 2°C or less. However, acceptable technological solutions that enable large scale CO2 storage or usage are still lacking. Therefore, NETs play a central role on national and international research agenda’s, including the mission-driven research programs of the UT in its ambition to ‘empower society through sustainable solutions’. c-COOL is a recently invented approach to provide safe CO2 storage based on the absorption of CO2 in clay nanoparticles (Univ. Twente 2019; patent pending). Clay nanoparticles, natural or nano-engineered for optimized performance, are loaded with CO2 and subsequently suspended in aqueous solutions for sequestration underground. Binding of CO2 to the clay nanoparticles minimizes the risk of CO2 leakage, which has been the source of substantial societal opposition against conventional CCS (carbon capture and storage) technologies. Scientifically and technologically the c-COOL research program focuses on the fundamental microscopic characterization and understanding of the clay-CO2 interaction, the loading and storage process, as well as subsequent processing and injection of suspensions into (model) porous media. The goal of the project is to provide a solid scientific basis to judge whether c-COOL can provide a significant contribution to the global fight against climate change.

The project is carried out by PCF in collaboration with the Sustainable Process Technology (SPT) and Separation Technologies (ST) groups within the UT and BP as major industrial partner.

Energy conversion and harvesting research
Solid-liquid interfaces are at the heart of many diverse processes related to energy harvesting, storage, and conversion, including battery technologies, electro- and photochemical conversion, and drop-based energy harvesting systems.

In collaboration with the Photochemical Systems group, PCF investigates the surface properties of semiconducting nanoparticles for applications e.g. in water splitting or solar-to-fuel applications. These particles display different surface charges and hydration structures on different crystalline facet, which lead to different chemical, photochemical, and electrochemical reactivity. High resolution AFM imaging and spectroscopy reveal details about these interactions and thereby improve our understanding of the performance of such nanoparticles in various photo-, electro- and general catalytic processes [Guo et al. 2018].

Drop-based energy harvesting has attracted substantial attention in recent years. The idea is based on the observation that water drops, such as rain drops, impinging onto solid surfaces often generate electrical signals between suitable electrodes embedded into the surfaces. This process thus directly converts mechanical energy of the falling drops into electrical energy. While numerous publications in high ranking journals celebrate the potential of this method the microscopic mechanisms underlying the process have remained rather elusive. To an important extent, this is due to the fact that the efficiency of the energy conversion process depends strongly on the presence of charge on the surface. That charge, however, is affected and in many cases even generated by the impinging droplets. As a consequence, charge generation and energy harvesting are difficult to disentangle. PCF has recently succeeded to separate these two steps by introducing and electrowetting-based charge injection process (EWCI) that allows to pre-charge the substrates in a well-defined manner [Wu et al. Small 2019; Wu et al. Adv. Mat. 2020]. Subsequently, synchronized high speed video imaging and electrical current measurements enabled a quantitative description of the mechano-electrical energy conversion process without any fit parameters [Wu et al. Phys. Rev. Lett. 2020 (in press)]. A theoretical analysis allowed to identify the previously injected charge density and the capacitance of the system as the relevant electrical parameters controlling the energy conversion efficiency (in addition to a variety of fluid dynamic parameters). While conversion efficiencies around 10% were achieved (and routes towards even higher efficiencies are clear) an analysis of the total available energy based on meteorological rainfall statistics reveals that even in allegedly rainy countries such as the Netherlands solar and wind energy are by far superior sources of renewable energy than rain. Nevertheless, island solutions in specific niches applications may well be of practical interest.

Semi-solid flow batteries are an innovative concept in which charge is stored in electrochemically active particles that are suspended in typically non-aqueous solvents. The particles can be charged and discharged in an electrochemical cell and pumped into much bigger external vessels for storage. Thereby, the storage capacity becomes independent of the dimensions of the electrochemical cell. To allow for efficient charging and discharging, the suspension contains next to the active particles and intercalating ions (typically Li+ or Na+) also electron-conductive carbonaceous material. The resulting colloidal suspension is displays a highly complex rheological and electrochemical response. PCF developed a dedicated rheo-impedance spectrometer that allows for a simultaneous characterization of both rheological and electrical properties of the suspension upon charging and discharging [Narayanan et al. Rev. Sci. Instr. 2019]. One of the key challenges identified during consecutive charging and discharging cycles was the gradual growth of a poorly conductive layer at the electrode-solution-interface, a so-called solid-electrolyte interphase (ESI) [Narayanan et al. ACS Appl Energy Mat. 2020].  


Experimental methods and tools: electrowetting, high resolution Atomic Force Microscopy and spectroscopy, (high speed) video microscopy, contact angle goniometry, particle tracking microrheology, macroscopic rheometry, (imaging) ellipsometry, quartz crystal microbalance, Langmuir trough and Langmuir-Blodgett deposition.