preparation-and-3d-tracking-of-catalytic-swimming-devices

Preparation and 3D Tracking of Catalytic Swimming Devices

We report a method to prepare catalytically active Janus colloids that "swim" in fluids and describe how to determine their 3D motion using fluorescence ...

Undesirable amorphous material generation during formulation is implicated in a growing number of pharmaceutical problems. Due to the importance of interfacial properties in many drug delivery systems, it seems that surface amorphous material is particularly significant. Consequently, this study investigates a range of methods capable of detecting and mapping surface amorphous material.

Identifying and mapping surface amorphous domains.

This study was conducted to accurately measure the dispersive surface free energy of lactose solids in ordered and disordered states.

Determination of the surface free energy of crystalline and amorphous lactose by atomic force microscopy adhesion measurement.

The elastic modulus of pharmaceutical materials affects a number of pharmaceutical processes and subsequently formulation performance and is currently assessed by bulk methods, such as beam bending of compacts. Here we demonstrate the accurate measurement of the elastic modulus of alpha monohydrate lactose from the dominant (011) face of single crystals using atomic force microscopy (AFM) as 3.45+/-0.90GPa. The criteria to ensure this data is recorded within the elastic limit and can be modelled using Hertzian theory are established. We compare and contrast this AFM method to a permanent indentation technique based upon a much larger Berkovich pyramidal indenter on a lactose compact and the wider literature. Finally the AFM was utilized to study the elastic response of amorphous lactose, demonstrating that the physical state of the amorphous material changes under repeated loading and behaves in a more crystalline manner under repeated force measurements, suggesting a pressure induced phase transition. The AFM based approach demonstrated has the significant advantages of requiring minimal sample, no need for producing a compact, being non-destructive in that no permanent indent is required and providing a technique capable of detecting variations in material properties across a single particle or a number of particles.

Elastic modulus measurements from individual lactose particles using atomic force microscopy.

A class of artificial microswimmers with combined translational and rotational self-propulsion is studied experimentally. The chemically fueled microswimmers are made of doublets of Janus colloidal beads with catalytic patches that are positioned at a fixed angle relative to one another. The mean-square displacement and the mean-square angular displacement of the active doublets are analyzed in the context of a simple Langevin description, using which the physical characteristics of the microswimmers such as the spontaneous translational and rotational velocities are extracted. Our work suggests strategies for designing microswimmers that could follow prescribed cycloidal trajectories.

Self-assembled autonomous runners and tumblers.

Eine Klasse von k&uuml;nstlichen Microswimmers mit kombinierten translatorischen und rotatorischen--einschlie&szlig;lich wird experimentell untersucht. Die chemisch heizten Microswimmers bestehen aus Dubletten von Janus kolloidale Perlen mit katalytischen Patches, die in einem festen Winkel zueinander positioniert sind. Die mittlere quadratische Verschiebung und die quadratisch gemittelte Winkelversatz von der aktiven Doubletten werden im Zusammenhang mit einer einfache Langevin-Beschreibung, durch die die physikalischen Eigenschaften von den Microswimmers wie dem spontanen translational analysiert und Rotations-Geschwindigkeiten werden extrahiert. Unsere Arbeit legt nahe, dass Strategien f&uuml;r das Entwerfen von Microswimmers, die Folgen k&ouml;nnten Zykloide Bahnen vorgeschrieben.

Monomolekularen autonome L&auml;ufer und Trinkgl&auml;ser.

Spin coating polymer blend thin films provides a method to produce multiphase functional layers of high uniformity covering large surface areas. Applications for such layers include photovoltaics and light-emitting diodes where performance relies upon the nanoscale phase separation morphology of the spun film. Furthermore, at micrometer scales, phase separation provides a route to produce self-organized structures for templating applications. Understanding the factors that determine the final phase-separated morphology in these systems is consequently an important goal. However, it has to date proved problematic to fully test theoretical models for phase separation during spin coating, due to the high spin speeds, which has limited the spatial resolution of experimental data obtained during the coating process. Without this fundamental understanding, production of optimized micro- and nanoscale structures is hampered. Here, we have employed synchronized stroboscopic illumination together with the high light gathering sensitivity of an electron-multiplying charge-coupled device camera to optically observe structure evolution in such blends during spin coating. Furthermore the use of monochromatic illumination has allowed interference reconstruction of three-dimensional topographies of the spin-coated film as it dries and phase separates with nanometer precision. We have used this new method to directly observe the phase separation process during spinning for a polymer blend (PS-PI) for the first time, providing new insights into the spin-coating process and opening up a route to understand and control phase separation structures.

In situ imaging and height reconstruction of phase separation processes in polymer blends during spin coating.

Nonconductive Janus particle swimmers made by coating fluorescent polymer beads with hemispheres of platinum have been fully characterized using video microscopy to reveal that they undergo propulsion in hydrogen peroxide fuel away from the catalytic platinum patch. The platinum coating shadows the fluorescence signal from half of each swimmer to allow the orientation to be observed directly and correlated quantitatively with the resulting swimming direction. The observed swimmer direction is consistent with both the bubble release and diffusiophoretic propulsion mechanisms.

Direct observation of the direction of motion for spherical catalytic swimmers.

The propulsion velocity of active colloids that asymmetrically catalyze a chemical reaction is probed experimentally as a function of their sizes. It is found that over the experimentally accessible range, the velocity decays as a function of size, with a rate that is compatible with an inverse size dependence. A diffusion-reaction model for the concentrations of the fuel and waste molecules that takes into account a two-step process for the asymmetric catalytic activity on the surface of the colloid is shown to predict a similar behavior for colloids at the large size limit, with a saturation for smaller sizes.

Size dependence of the propulsion velocity for catalytic Janus-sphere swimmers.

In this paper we show that processes such as Brownian motion, convection, sedimentation, and bacterial contamination can cause small particles to move through liquids in a fashion which may be mistaken as nanopropulsion. It is shown that particle tracking and subsequent statistical analysis is essential to ascertain if small particles actually propel themselves, or if they are propelled by another process. Specifically we find that it is necessary to calculate the mean-squared displacement of particles at both short and long time intervals, to show that the direction of propulsion changes coincident with rotation of the particle by Brownian motion, as this allows nanopropulsion to be differentiated from Brownian motion, convection and sedimentation. We also find that bacteria can attach themselves to particles and cause them to be propelled. This leads to motion which appears very similar to nanopropulsion and cannot be differentiated using particle tracking and therefore find that carefully designed control experiments must be performed. Finally, we suggest an experimental protocol which can be used to investigate the motion of small objects and prove if they move due to nanopropulsion.

Importance of particle tracking and calculating the mean-squared displacement in distinguishing nanopropulsion from other processes.

In this work, we show that the asymmetrical distribution of mass at the surface of catalytic Janus swimmers results in the devices preferentially propelling themselves upward in a gravitational field. We demonstrate the existence of this gravitaxis phenomenon by observing the trajectories of fueled Janus swimmers, which generate thrust along a vector pointing away from their metallically coated half. We report that as the size of the spherical swimmer increases, the propulsive trajectories are no longer isotropic with respect to gravity, and they start to show a pronounced tendency to move in an upward direction. We suggest that this effect is due to the platinum caps asymmetric mass exerting an increasing influence on the azimuthal angle of the Janus sphere with size, biasing its orientation toward a configuration where the heavier propulsion generating surface faces down. This argument is supported by the good agreement we find between the experimentally observed azimuthal angle distribution for the Janus swimmers and predictions made by simple Boltzmann statistics. This gravitaxis phenomenon provides a mechanism to autonomously control and direct the motion of catalytic swimming devices and so enable a route to make autonomous transport devices and develop new separation, sensing, and controlled release applications.

Gravitaxis in Spherical Janus Swimming Devices.

Spin-coating offers a facile fabrication route for the production of high quality colloidal crystals, which have potential as photonic band-gap materials. This paper presents the results of direct observations of the self-assembly of latex colloids during spin-coating through the use of stroboscopic microscopy. We have been able to identify several mechanisms by which self-assembly occurs, depending upon the dispersion properties, such as particle weight fraction, solvent volatility and viscosity. Through the use of stroboscopic microscopy we have directly observed ordering occurring due to high concentrations of colloid particles (where volatility is relatively low), resulting in the formation of regular close packed ordered particle arrays. Conversely when the system in spun-cast from a much more volatile solvent, highly disordered non-equilibrium arrangements of particles form. When spin-coating a low concentration, low volatility dispersion, ordering is dominated by the occurrence of capillary drying fronts, which drag the particles into ordered arrangements. At a volatility intermediate to that of water and ethanol, ordering occurring predominantly via shear forces. Finally when the volatility is increased beyond the shear ordering regime, excessive shear leads to the occurrence of drying fronts within the system and so again, capillary forces induce a large degree of order within the final film.

On the mechanisms of colloidal self-assembly during spin-coating.

The mode of lysozyme protein adsorption at end-tethered thiol-terminated polyethylene oxide brushes grafted upon gold was determined in situ by neutron reflectivity using the INTER instrument at target station 2, ISIS, RAL, UK. It was found that the most probable position of protein adsorption at these weakly protein resistive brushes was at the gold-brush interface in the so-called primary protein position.

Mode of lysozyme protein adsorption at end-tethered polyethylene oxide brushes on gold surfaces determined by neutron reflectivity.

The advent of autonomous self-propulsion has instigated research towards making colloidal machines that can deliver mechanical work in the form of transport, and other functions such as sensing and cleaning. While much progress has been made in the last 10 years on various mechanisms to generate self-propulsion, the ability to steer self-propelled colloidal devices has so far been much more limited. A critical barrier in increasing the impact of such motors is in directing their motion against the Brownian rotation, which randomizes particle orientations. In this context, here we report directed motion of a specific class of catalytic motors when moving in close proximity to solid surfaces. This is achieved through active quenching of their Brownian rotation by constraining it in a rotational well, caused not by equilibrium, but by hydrodynamic effects. We demonstrate how combining these geometric constraints can be utilized to steer these active colloids along arbitrary trajectories.

Article	Year
Identifying and mapping surface amorphous domains. Pharmaceutical research\| PubMed ID: 16028021	2005
Determination of the surface free energy of crystalline and amorphous lactose by atomic force microscopy adhesion measurement. Pharmaceutical research\| PubMed ID: 16421667	2006
Elastic modulus measurements from individual lactose particles using atomic force microscopy. International journal of pharmaceutics\| PubMed ID: 17074456	2007
Self-assembled autonomous runners and tumblers. Physical review. E, Statistical, nonlinear, and soft matter physics\| PubMed ID: 20866681	2010
In situ imaging and height reconstruction of phase separation processes in polymer blends during spin coating. ACS nano\| PubMed ID: 21561158	2011
Direct observation of the direction of motion for spherical catalytic swimmers. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 21928845	2011
Size dependence of the propulsion velocity for catalytic Janus-sphere swimmers. Physical review. E, Statistical, nonlinear, and soft matter physics\| PubMed ID: 22463141	2012
Importance of particle tracking and calculating the mean-squared displacement in distinguishing nanopropulsion from other processes. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 22731393	2012
Gravitaxis in Spherical Janus Swimming Devices. Langmuir : the ACS journal of surfaces and colloids\| PubMed ID: 24134682	2013
On the mechanisms of colloidal self-assembly during spin-coating. Soft matter\| PubMed ID: 25269118	2014
Mode of lysozyme protein adsorption at end-tethered polyethylene oxide brushes on gold surfaces determined by neutron reflectivity. The European physical journal. E, Soft matter\| PubMed ID: 25743024	2015
Boundaries can steer active Janus spheres. Nature communications\| PubMed ID: 26627125	2015

Stephen Ebbens

Biography

JoVE Journal Publications

Other Publications