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.
When a soil is subject to low temperatures, the freezing of water can lead to an upward forces and the deformation of the soil with the growth of ice. This phenomena, called frost heaving can lead to catastrophic destruction of roads or buildings in cold regions as shown in figure above. The most obvious explanation for this phenomenon comes from the fact that water expands by about 9% when
it turns into ice. If the water completely fills the porous soil and starts freezing, the ice will subsequently apply pressure on the soil leading to its deformation. This explanation falls short however, as soils are generally not fully saturated and one would then expect the ice to be able to grow where there is room to expand. It is believed that the build-up of ice happens largely because
water in the unfrozen soil below gets drawn up into the freezing zone and attaches itself to the existing frost crystals to form ever thickening layers of ice. The phenomenon is also known as ice segregation or ice lense formation. The cause of frost heave is therefore somewhat not fully understood, and a lot of questions remain unanswered.
During this master project, you will experimentally investigate the mechanism of phase transition of water to ice in granular materials when partially saturated with water. The objective is to understandwhat the conditions for ice lenses formation are. For that, you will design an experimental set-up similar to a Hele-Shaw cell to study a model soil under freezing conditions. Several parameters will beinvestigated such as:
- the saturation of the soil,
- its physical properties (hydrophilicity, compaction and poydispersity)
- the freezing temperature
This master research project is a collaborative project between Weber Beamix and the Soft Matter group at the IoP- UvA. The research will be done in the Soft Matter group at the University of Amsterdam in close collaboration with the R&D department of Weber Beamix. During the project, the student will visit the company to become familiar with their different activities.
Gypsum efflorescence is one of the major problems affecting clay brick masonry. Efflorescence consists of the crystallization of various soluble salts at the surface of porous materials (bricks and stones, etc.), appearing as a white-grey stains months or several years after the construction. Gypsum efflorescence adheres very strongly to the rough surface of the bricks and is very difficult to remove because of the low solubility of calcium sulfate crystals. The appearance of gypsum stains can occur on facades after exposure to water and rain followed by evaporation due to environmental fluctuations, i.e. low relative humidity and/or higher temperature.
The objective of this project is to find an ecologically friendly solution for the easy removal of gypsum efflorescence from stones, i.e., rough and porous surfaces. For this purpose, different additives (surfactants and anticaking agents) having the role of nucleation promoters and growth modifiers will be tested with the intention to suppress the adhesive properties of the precipitated gypsum at the surface. Similar to the case of caking the formation of solid bridges between the crystals and the surfaces during the evaporation of thin liquid films can increase the adhesive properties of the salt crystals. The main objective is then to decrease the effective contact area between crystals and the surface and, on the other hand, improves the spreading ability of the water over the crystalline surfaces in order to avoid solid crystalline bridge between crystals and surface.
The student will therefore perform multiscale Experiments: at the microscale precipitated gypsum from evaporating droplets will be exposed to water with different types of additivesand made to evaporate again to induce recrystallization; the kinetics of dissolution/recrystallization will be followed under phase contrast microscopy and confocal Raman microscopy.
At the macroscale, Gypsum-contaminated model stones (limestone and sandstone) will be prepared and exposed to the same water and additives. The drying kinetics and the recrystallization process will be studied using various techniques such as NMR, SEM, and weight measurements. Micro X-ray Tomography analysis on some selected stone samples can also be planned in collaboration with our partner at the University of Pau in France.
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.
biomaterials with tunable elastic properties. We have recently discovered that hydration dramatically affects the mechanical properties of the final collagen material. This provides a new method to design functional materials with tunable mechanical properties. Specific macroscopic properties may be obtained by adapting the environment instead of perfecting the chemistry of the biomaterial’s building blocks. In this project, you will explore the effect of different solvents on the molecular and macroscopic properties of collagen using infrared spectroscopy, imaging techniques and rheology
How nature optimizes self-assembly and elastic properties of collagen. Although Type 1 collagen is found in most connective tissues, OI mutations mostly affect bones, whereas little or no effect is observed in other tissues such as skin and blood vessels. This suggests that in these tissues compensating mechanisms take place, which minimize the impact of OI mutations. Such compensatory mechanisms might be related to the fact that in all other connective tissues collagen interacts with other biomacromolecules such as hyaluronic acid. This interaction might compensate for the collagen defects at the molecular level, including altered collagen hydration. In this project, you will investigate the effect of hyaluronic acid on the collagen hydration and assembly by using imaging techniques, infrared spectrosocpy, rheology and turbidity experiments.
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
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.
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.
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.