If one fills a glass with water, the fluid stream can create air entrainment: bubbles of air form in the glass. Air entrainment in liquids is a complex phenomenon that has important applications in industry and environment. For example, in naval hydrodynamics, the breakup of entrained air into bubbles leads to acoustic noises which modifies their hydrodynamic performances. In industrial processes the phenomenon is widely encountered and can be either a desired or detrimental effect: the pouring of molten glass or steel into a mold can lead to the formation of air bubbles which can be disastrous for the mechanical properties of the steel, whereas when using dishwashing liquid the formation of bubbles is clearly desired.
In the project we perform an experimental study using high-speed photography of the plunging of a liquid jet into a pool of the same liquid. The entrainment of the surrounding air then forms a stream of bubbles in the receiving pool. This process depends on a variety of system parameters including the fluid characteristics such as the viscosity, surface tension and the jet parameters such as the jet diameter and its flow speed. Previous research has shown that there is a critical value for the jet velocity, called minimum entrainment velocity, for which air entrainment occurs (see figure); however what determines the critical value remains ill-understood; understanding this will be one of the central aims of the project.
Nanocrystals offer fascinating properties and application perspectives for new photovoltaic devices: due to their small size, quantum effects become important, leading to discretized energy levels (-> electron in a box problem) just like atoms have. The advantage of nanocrystals is that we can set the energy levels by tuning their size, and we can assemble them into new “supersolids”, which which due to overlap of the energy levels show a new band structure, just like the band structure of regular solids. Because of the tunability of energy levels and electronic states, this offers tremendous potential for research and optoelectronic applications. The electronic states can be tailored by the strength of the nanocrystal interaction within the assembled “supersolid”, which is in principle determined by the QD distance. So far, however, this has remained a largely theoretical concept, and experimental realization of such tailored “quantum solids” has remained a grand challenge.
Here we take advantage of recently synthesized cubic all-inorganic perovskite QDs to demonstrate the full transition from a single dot of a core-shell structure, via weakly and strongly coupled quantum solids to a fully joint solid thin semiconductor layer, where due to fusion of the nanocrystals, the quantum confinement vanishes. These novel perovskite NCs offer the great advantage that (i) their cubic shape boosts self-assembly into highly-ordered two-dimensional square lattices enhancing mutual coupling, and (ii) they merge naturally by a diffusive mechanism. The envisaged core-shell structure will open new routes to:
Tailor the inter-nanocrystal separation, and thus the coupling within the square superlattice.
Realize continuous in-situ tuning of electronic states through in-situ expansion of the nanocrystal core at the expense of the shell.
The outcome will be the bottom-up design of highly tunable novel materials for efficient energy conversion
Supervisor: Cees van Rijn and Daniel Bonn
More info: firstname.lastname@example.org
Project type: Bachelor or Master
Nozzles with a non-circular cross-section are able to produce spontaneously droplets with a droplet size that is not dependent on surface tension, shear or gravity forces. Such a nozzle is highly advantageous for e.g. 3D printing where currently the production of small droplets requires the use of piezo elements to enforce the formation of small droplets. This makes droplet formation more complicated and droplet size dependent on surface tension and shear. In earlier work on non-circular nozzles we observed that different designs lead to different spontaneous droplet formation processes. The goal of this Master project will be to: 1. Learn about microfluidics and droplet formation techniques 2. Modelling droplet formation (using analytical models) while taking into account the nozzle geometry 3. Studying droplet formation experiments while taking into account the nozzle geometry 4. Show that you can design a nozzle geometry, in that surface tension, shear or gravity has only a limited influence on the droplet size 5. Study if such a nozzle can be applied in one or more health applications
Understanding the spreading of liquid drops on planar substrates is important in various applications (spraying, agriculture, painting and printing …) in which the dynamics of moving contact linesplays a major role. It involves the surface energies of all interfaces and hence the wettability of the materials. Surprisingly, Droplets spreading is observed on hydrophobic surfaceswhen both salt and surfactant are present in the solution.
This project consists of:
1 - Experimentally Measuring the spreading properties at different concentrations of different salts and surfactants.
2 - Studying the role of the wettability of the substrates.
3 - Quantifying the dynamics of the moving contact line by image analysis.
4 - Understand the role of NaCl on surfactant-surface interactions during droplet spreading.
Over the past few years, exciting progress has been made in the field of mechanical metamaterials. Harnessing nonlinear degrees of freedom arising in suitably designed microstructures, metamaterials could be programmed with specific mechanical tasks, such as negative stiffness, elastic hysteresis or programmable mechanics (1-4). So far, most of these developments have been made with passive-at equilibrium-materials. .
The goal of this project is generalize these findings to create a new class of programmable, dynamical and active materials, called Machine Materials. To do this, the idea is to set the grounds of nonlinear osmomechanics, which combines swelling and nonlinear elasticity within metagels, i.e. architected hydrogels under osmotic shock.
Florijn, Coulais, van Hecke, Programmable Mechanical Metamaterials. Phys. Rev. Lett. 113, 175503 (2014)
Coulais, Overvelde, Lubbers, Bertoldi and van Hecke, Discontinuous Buckling of Wide Beams and Metabeams. Phys. Rev. Lett. 115, 044301 (2015)
Coulais, Teomy, de Reus, Shokef and van Hecke, Combinatorial Design of Textured Mechanical Metamaterials. Nature 535, 529-532 (2016)
Coulais, Sounas, and Alù, Static Non-Reciprocity in Mechanical Metamaterials. Nature, 542, 461-464 (2017)
Colloidal particles offer fascinating insight into the statistical mechanics and assembly behaviour of atoms. The particles, about a micrometer in size, have thermal energy and attractive/repulsive interactions similar to atoms, making them form phases very similar to their atomic counterpart. Yet, as these colloidal particles are larger, they can be easily observed in real space and time. Besides being atomic models, these particles serve as building blocks for the assembly of new micro- and nanoscale materials that are used, e.g. in photonics and optoelectronics.
Recently, we succeeded in assembling analogues of molecules using “patchy” colloidal particles. These particles interact via attractive patches in specific directions only, making them form structures known from molecular compounds. The movie shows a pair of particles with four patches, clearly bonded via one of their patches (bright). Excitingly, such 4-patch particles form structures known from molecular carbon, such as carbon rings C5 and C6, and more complex molecules such as butane and butene.
Using these colloidal molecules, we obtain unique insight into their atomic counterpart by 3D reconstruction of their structure. We analyse their vibration spectrum, conformations and relaxation, and how this depends on the patch attraction, which we can vary continuously. In this project, the student can explore assembling different molecular compounds, and study their formation and relaxation, thus obtaining insight into the molecular dynamics.
The surface energy of a solid influences the growth rate, adsorption, catalytic behavior, surface segregation and the formation of grain boundaries. Their determination is of great importance for understanding mechanisms of many physical phenomena. Despite its importance, surface energy values are very difficult to measure experimentally although computer simulation results can be found.
This project consists of:
1 - Setting up an experiment for the control growth of pendant NaCl crystals at the liquid/air interface.
2 - Record and analyse the time evolution of the crystal growth till it falls in the solution.
3 - Scanning Electron Microscopy (SEM) analysis of the pendant crystals at different stages of growth
4 - Estimating the surface energy of the crystal by analogy to the pendant drop method for surface tension measurements of liquids.
In recent years, few-layered transition metal dichalcogenides have shown a rise in popularity due to their interesting optical properties, such as high exciton-binding energies, rich electronic structure and relatively strong optical response. However, their applications have been limited by the difficulty of synthesizing high-quality samples on a large area, hindering their use in photovoltaics for example. To date epitaxial techniques such as chemical vapor deposition (CVD) provide the samples of highest quality but this technique is time-consuming and still not very scalable. In the field of nanocrystal quantum dots, self-assembly techniques have been used to synthesize novel superstructures. One of these techniques, deposition by critical Casimir forces has been used to mimic the epitaxial growth of thin films starting from nanocrystals in solution, requiring narrow size-distributions of the starting nanocrystals. The picture above sketches the idea: Deposition of quantum dots on a substrate is achieved by critical Casimir forces via small temperature variations.
In this work we explore the method of deposition by critical Casimir forces on MoS2 quantum dots to fabricate few-layer films of MoS2 and follow the evolution of its optical properties from a 3D confined system to a 2D confined system and compare the final product to MoS2 of similar thickness produced by the more traditional techniques of CVD and atomic layer deposition (ALD).
You will start in the chemistry lab with the starting nanocrystals and do some initial size-selection steps by centrifugation. Using a pre-existing setup you can deposit the particles in layers after which both imaging and optical characterization techniques will be used in our optics lab to characterize the samples and further optimize the procedure.
In our daily lives (and in physics books) pressure is generally a positive quantity. But there is no fundamental reason against having negative pressure: instead of compressing a liquid (=positive pressure) you simply have to stretch it. Just put some water in an airtight cylinder and pull on the plunger! Of course, normally when you do this, the water starts to form vapor bubbles until the pressure of the liquid+vapor system is back to ambient (and thus positive). However, the vapor bubbles can only form by nucleation, typically on small contaminations in the liquid. Hence, in super-clean water, bubble formation does not occur (it is for this reason that you can heat clean water above 100°C without it starting to boil), and so negative water pressure becomes feasible. Actually experiments [1,2,3] have shown that water can exist at negative pressures down to –1000 bar! (for comparison, the pressure in a normal bicycle tire is about +2 bar).
So far, very little is known about the properties of water at negative pressure. These properties are expected to be unique, because the behavior of water at negative pressure is dominated by the attractive forces between the molecules, rather than the short-range repulsive forces that dominate the behavior of liquids at normal, positive pressure.
In this project you will explore new ways of creating water at negative pressure, and you will use spectroscopic methods to investigate its properties, both macroscopic and at the molecular level.
 Zheng et al. Liquids at large negative pressures: water at the homogeneous nucleation limit. Science 254, 829 (1991).
 Green et al. Water and Solutions at Negative Pressure: Raman Spectroscopic Study to -80 Megapascal. Science 249, 649 (1990).
 Azouzi et al. A coherent picture of water at extreme negative pressure. Nature Physics 9, 38 (2013).
Layered two-dimensional materials have strong in-plane bonding but weak out-of-plane bonding. This allows them to be peeled layer by layer, literally with regular household tape.
In recent years, the “mechanical exfoliation” of layered materials using scotch tape, as in graphite/graphene and MoS2, has yielded a novel class of 2D materials with exciting optical and electronic properties, and spurred the search for even further layered compounds. Among the many systems investigated are the transition metal dichalcogenides, transition metal tri-chalcogenides and post-transition metal trichalcogenide semiconductors. Belonging to this last group is Bi2S3, a direct-bandgap (`~1.4 eV) semiconductor showing photolumi-nescence in the visible region in nanocrystal form as prepared by solution-phase methods. However, no reports have been made yet on direct mechanical exfoliation of this compound and the resulting optical properties. In this project, we attempt to mechanically exfoliate bulk Bi2S3 and characterize its optical properties. In addition, early reports on liquid exfoliation of bismuth sulphide show that exfoliation results in reduced sulphide stoichiometry and as such we apply the TFSI superacid treatment, previously successful on MoS2, to reduce sulphide vacancies and enhance the photoluminescence properties of exfoliated layers of Bi2S3.
You will work on so-called mechanical exfoliation by scotch-tape method in the chemistry lab, after which a chemical treatment is applied. After that, you will measure the optical properties of this material in our optics lab and analyse the data to learn about the fundamental photo-physical mechanism in this new class of materials.
Complex crystalline alloys are extremely important as materials with specific mechanical and electrical properties and melting/ solidification behaviour. Yet, their solidification and mechanics are difficult to study at the atomic scale. Recently, we have assembled analogues of these materials using colloidal particles, i.e. micron-size particles that can be engineered with exquisite control over size, shape, and mutual attractions. These colloidal building blocks can be made with finely tunable interactions so that they form complex structures just like atoms do, but on a much larger scale, making them directly observable in real-space and time. Besides being models for atoms, the particles also serve as new building blocks for novel nano- and micro structured materials used in photonics and optoelectronics.
The goal of this project is to make crystalline alloys from two types of particles, A and B, and investigate their formation. Just like in atomic alloys, mixing particles A and B of certain size ratio, and relative attraction (interaction energies uAA, uBB, and uAB) gives rise to specific crystal structures. We have a large collection of A and B particles available with specific size ratio and relative attraction that we can even vary with temperature. This combination allows to explore building new crystal structures, and by investigating the process, obtain unique insight into the crystallization kinetics of these complex crystals, which is inaccessible in atomic alloys. Because the particle sizes are of the order of a micrometer, we can image the individual particles in three dimensions using a powerful optical microscopy technique known as confocal microscopy. This technique allows following the individual particle trajectories in three-dimensional space. The image below shows a crystalline microstructure of a strongly attractive particle A (bright) with a less attractive particle B (faint blue), “quenched” in a complex colloidal microstructure. Such direct-space observation provides unique insight into basic mechanism of (atomic) assembly.