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An electrode brought to the surface of a liquid that contains microparticles can be used to pull out surprisingly long chains of particles. Curiously enough, the particles in the chains are held together by a thin layer of liquid that covers them. The phenomenon visually resembles one of the most important modern-day technological processes - namely, Prof. Jan Czochralski's method of growing monocrystals - which involves slowly pulling a rod-mounted seed crystal out of molten material. The materials that are fabricated using Czochralski's method are crystals, so their structure is regular in all three dimensions. We create our structures using not atoms or molecules but microparticles arranged regularly along only one dimension. With a bit of leeway, we might therefore treat our chains as one-dimensional crystals. This spectacular phenomenon discovered in our group holds promise for a broad variety of applications, including in electronics.

This work was published in Nat. Commun. 8, 15255 (2017) link


The 1D particle structures of microparticles are formed as a result of complex interactions of an electrical, gravitational, and capillary nature. Here, gravity plays the role of a spoiler: if a chain gets too heavy, gravity will cut it like scissors. There is much to indicate that such chains could also be formed under zero gravity and could be then of practically any length. The length of the particle chains depends on the number and weight of the microparticles, which is usually closely related to their size. We conducted experiments for particles with diameters ranging from around 100 nanometers to 200 micrometers. Chains formed from micron-sized particles had up to several thousand elements and were up to several centimeters long. Viability over a wide range of particle size: (a) ~100 nm, (b) ~15 um, (c) ~25 um, (d) ~55 um, (e) ~100 um, (f) ~200 um.

This work was published in Nat. Commun. 8, 15255 (2017) link


Once the 1D particle structures are formed, they behave like chains: they are flexible, meaning they can be bent into various shapes. However, much depends on the type of liquid used. In some experiments, we pulled chains out of molten paraffin wax. Shortly after being pulled, the bridges would solidify and the structure would become rigid. There's also an intermediate option: if we blend, for example, resin and alcohol, the resin hardens as the alcohol evaporates. The chain is then a lot less flexible but not completely rigid. The chains formed via our electric route remain stable even after the external field is turned off, held together by the sphere-sphere liquid bridges, which at the same time render them flexible enough to bend. This combination of properties makes it possible to subsequently deposit the formed chains into any targeted patterns via direct writing.

This work was published in Nat. Commun. 8, 15255 (2017) link


We've all probably seen our mothers or grandmothers wear necklaces made of beads strung on a thread. The chains of microparticles fabricated and studied by our team look very similar, but they are much smaller in size. However, the most interesting thing is the physics behind this process. The formation of these regular structures is determined by a set of phenomena that are by no means trivial, and the role of the thread that holds together the individual beads is played by... a liquid. What is more, in a necklace, the thread goes through the beads, while our thread, in this case the liquid, actually covers the microparticles.

This work was published in Soft Matter 13, 4698-4708 (2017) link


Capillary bridges can be used for fabricating new materials and structures. Here, we describe theoretically and validate experimentally the mechanism of formation of capillary bridges during a process in which a beaded chain is being pulled out from a liquid with a planar surface. There are two types of capillary bridges present in this system, namely the sphere-planar liquid surface bridge initially formed between the spherical bead leaving the liquid bath and the initially planar liquid surface, and the sphere-sphere capillary bridge formed between neighbouring beads in the part of the chain above the liquid surface. During the process of pulling the chain out of the liquid, the sphere-planar liquid surface bridge transforms into the sphere-sphere bridge. We show that this transition can be either continuous or discontinuous. The transition is continuous when the diameter of the spherical beads is larger than the capillary length.

This work was published in Soft Matter 13, 4698-4708 (2017) link


Crumpling of deformed particle shells formed on droplets. Wrinkling of a particle shell occurs at regions where the compressive stress surpasses the elastic tension of the shell and the surface tension of the droplet interface. In this work, we showed that the application of an electric field can be used to probe the thickness and bending stiffness of particle shells formed on emulsion droplets through induced wrinkling of the shell layers. We demonstrated that on a curved interface, multiple wrinkles are formed with a characteristic wavelength that increased for shells composed of larger particles and formed on larger droplets.

Published in ACS Appl. Mater. Interfaces, 11, 29396-29407 (2019) link


Electric field-induced crumpling of the particle shell formed on a droplet. Particle shell formed on a 2-mm silicone oil droplet composed of polyethylene particles (~3 um). Initially, in the absence of an electric field, the shell was spherical. The shell was then subjected to an electric pulse (1 second), which deformed the shell. After the electric field was turned off, the shell relaxed back toward a spherical shape, and the particle film crumpled. Wrinkles vanished as the particle shell relaxed and covered the entire droplet again.

Published in ACS Appl. Mater. Interfaces, 11, 32, 29396-29407 (2019) link


The process of opening and closing the particle shell visually resembles the expansion and contraction of a human eye's pupil or an optical diaphragm. This inspired us to demonstrate the use of particle-covered droplets as a millimeter-sized diaphragm with an adjustable aperture for controlling the light passage. When the droplet was subjected to a DC electric field, an opening in the particle film was formed at the droplet's electric pole, and the laser light could pass through the droplet. The droplet relaxed to a spherical shape after we turned off the electric field, and the particles covered the entire droplet again. Consequently, the laser light was blocked again.

Published in ACS Appl. Mater. Interfaces 11, 25, 22840-22850 (2019) link


Silicone oil droplet (~3 mm) covered with polyethylene particles (~20 um) viewed perpendicular to the electric field direction. The droplet was surrounded with castor oil and docked in an O-ring that was fastened to one of the electrodes. In the presence of a DC electric field, the electric stress deformed the droplet. As a result, the droplet's surface area increased, allowing particles to unjam and electrohydrodynamic flows to arise. The EHD flows conveyed particles away from the droplet's electric pole, forming a particle-free area. Active, tunable, and reversible opening and closing of particle shells on droplets may facilitate chemical reactions in droplets and enable various small-scale laboratory operations, including online detection, measurement, and adjustment of droplet liquid.

Published in ACS Appl. Mater. Interfaces 11, 25, 22840-22850 (2019) link


This movie shows the process of forming a droplet covered with densely packed particles. The convective electrohydrodynamic liquid flows enable the formation of a monolayer film composed of densely packed particles arranged in a nearly hexagonal geometry. Silicone oil droplet covered with polyethylene red particles (average size ~50 um) were made inside the castor oil using a micropipette. The particles were transported at the silicone oil droplet interface by applying a DC electric field.

Published in ACS Appl. Mater. Interfaces 11, 25, 22840-22850 (2019) link


We proposed a new bulk approach to fabricating Pickering emulsions by electric fields. We prepared oil-in-oil emulsions stabilised by microparticles and controled the mean size of the Pickering droplets. In our approach we took advantage of total surface area reduction of emulsion droplets by electrocoalescence. This led to an increase in particle coverage, and eventually to formation of densely packed particle shells on Pickering droplets. First, we prepared an unstable pre-emulsion with droplets having small sizes and low particle coverages, from which the final Pickering emulsion was formed via consecutive coalescence events sped up by application of electric field.

This work was published in Soft Matter 14, 5140-5149 (2018) link


Actively arranging surface-adsorbed conductive particles by switching the electric field frequency from 0 Hz (EHD flow regime) to 200 Hz (particle dipolar interaction regime). A silicone oil drop withdiameter ~3 mm is viewed (left) along and (right) perpendicularto the electric field direction. Ag-coated glass microspheres with mean diameters of ~20 um were used in this experiment. The structuring process is reversible, and the switching between two particle conformations occurred in seconds. We also observed that particles at the droplet's surfaces subjected to a constant DC electric field exhibit different structuring depending on their electrical properties and coverage.

This work was published in Soft Matter 14, 5442-5451 (2018) link


A designed assembly of particles at liquid interfaces offers many advantages for development of materials, and can be performed by various means. Electric fields provide a flexible method for structuring particles on drops, utilizing electrohydrodynamic circulation flows, and dielectrophoretic and electrophoretic interactions. In addition to the properties of the applied electric field, the manipulation of particles often depends on the intrinsic properties of the particles to be assembled. Slightly more electrically conductive polystyrene particles structure into chains, whereas the pristine polystyrene particles are moved towards electric poles of a droplet.

This work was published in Materials 10(4), 329 (2017) link


Two silicone oil droplets formed in castor oil. The droplets' surfaces are covered with microparticles. An external DC electric field is applied in the horizontal direction. Driven by the attractive EHD and electrostatic forces, the drops approach one another allowing their coalesce. After coalescence occurs, a Janus shell is formed rapidly in a few seconds. The coalesced drop is now fully covered by particles. During the coalescence, the particles can move and reorganize only within the drop surfaces due to the strong desorption energies. The particles on the droplet's surface are closely packed and arranged in slightly disordered hexagons. Turning off the electric field does not induce any observable movement of the particles in the shell, that is, the shell is stable.

This work was published in Nat. Commun. 5, 3945 (2014) link


Janus, the old Roman god of beginnings and transitions, attracted believers' attention with his two faces, each looking to different direction of the world. Janus capsules made up of two shells stuck one another, each composed of micro- or nanoparticles of different properties - have been for some time attracting the researchers' attention. They see in such capsules an excellent tool for transporting drugs and a vehicle leading to innovative materials. To have, however, Janus capsules generally accessible, efficient methods for their mass production must be developed. In our work, we present a method of fabricating patchy shells on droplets.

This work was published in Nat. Commun. 5, 3945 (2014) link


At low particle coverage on droplet's surface, increasing the electric field strength induces an instability of the particle ribbon, which breaks up into several counter-rotating colloidal assemblies. The number of domains depends on the strength of electrohydrodynamic flows of liquids, and the freqency of the domain rotation is proportional to the electric field strength. In our follow-up research we demonstrate the importance of the electrical properties of particles in their assembly on the droplet's surface and the reduction of convective flows of liquids induced by an electric field.

This work was published in Nat. Commun. 4, 2066 (2013) link


Colloidal particles are carried by electrohydrodynamic (EHD) convective flow, concentrating the particles in a dense packed colloidal ribbon on the droplet's surface. This is reminiscent of the 'coffee-ring' effect, which is owing to colloidal particles carried by capillary convective flow in an evaporating droplet. The formation of the ribbon involves two processes: transport of particles from the droplet's bulk to the droplet's surface and assembly of the ribbon film on that surface. Two last movies present assembly of particles that are already at the surface of a droplet. Viewed along and across the electric field direction.

This work was published in Nat. Commun. 4, 2066 (2013) link


The seed for our lab growing was planted in 2013. Thanks to the HOMING grant from the Foundation for Polish Science (FNP), I was able to purchase tools and materials to build a low-budget experimental setup and offer stipends for MSc students. The first experimental setup was built in the Institute of Physical Chemistry at the Polish Academy of Science where I worked for two years. Thanks to prof. Piotr Garstecki and the members of his group I grew up as a scientist. My fully independent scientific career begun in 2015, where I was offered a lab and office space (at the Dapartment of Physics AMU, Poznan) to develop the group and speed up the scientific activities. Here, I would like to thank my Ph.D. supervisor prof. Jon Otto Fossum for introducing me to the world of the fundamental science. In this video I talk about the fabrication of particle shells on droplets and the scholarship received from the FNP.