The evaporation of liquid droplets containing colloids is an omnipresent natural phenomenon that has received much attention due to the fundamental effects it entails, as well as the multitudes of fields in which it can be applied. The deposition of particulates onto a solid surface during evaporation tends to form ring‐like stains, which are a hallmark of the “coffee‐ring effect.” A wide variety of systems has already been employed to suppress or enable this effect, however, little attention has been focused on particles in restricted geometries that are driven far from equilibrium. Here, we investigate how self‐propelled, “active” catalytic Janus microspheres affect the ring stains left behind during the drying of a geometrically confined suspension containing such particles. Self‐propulsion results indirectly from the decomposition of hydrogen peroxide (H2O2) on the catalytically active hemispherical shell, while the diametrically opposite face is inert; this is how the system is driven out of equilibrium. The magnitude of activity can be controlled by adjusting the volume concentration of aqueous H2O2within the suspension. This parameter strongly influences the ring‐shaped microstructures obtained, especially when the concentration is sufficiently high to produce oxygen bubbles that take over the motion as opposed to auto‐phoresis.
Liquid–liquid or liquid–air interfaces provide interesting environments to study colloids and are ubiquitous in nature and industry, as well as relevant in applications involving emulsions and foams. They present a particularly intriguing environment for studying active particles which exhibit a host of phenomena not seen in passive systems. Active particles can also provide on‐demand controllability that greatly expands their use in future applications. However, research on active particles at interfaces is relatively rare compared to those at solid surfaces or in the bulk. Here, magnetically steerable active colloids at liquid–air interfaces that self‐propel by bubble production via the catalytic decomposition of chemical fuel in the liquid medium is presented. The bubble formation and dynamics of “patchy” colloids with a patch of catalytic coating on their surface is investigated and compared to more traditional Janus colloids with a hemispherical coating. The patchy colloids tend to produce smaller bubbles and undergo smoother motion which makes them beneficial for applications such as precise micro‐manipulation. This is demonstrated by manipulating and assembling patterns of passive spheres on a substrate as well as at an air–liquid interface. The propulsion and bubble formation of both the Janus and patchy colloids is characterized and it is found that previously proposed theories are insufficient to fully describe their motion and bubble bursting mechanism. Additionally, the colloids, which reside at the air–liquid interface, demonstrate novel interfacial positive gravitaxis towards the droplet edges which is attributed to a torque resulting from opposing downward and buoyant forces on the colloids.
more » « less- Award ID(s):
- 2218980
- NSF-PAR ID:
- 10438753
- Publisher / Repository:
- Wiley Blackwell (John Wiley & Sons)
- Date Published:
- Journal Name:
- Advanced Materials Interfaces
- Volume:
- 10
- Issue:
- 31
- ISSN:
- 2196-7350
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Studying the behavior of anisotropic particles at fluid interfaces is a rapidly expanding field, as understanding how the introduced anisotropy affects the resulting properties is essential in the engineering of interfacial systems. Surface anisotropic particles, also known as Janus particles (JPs), offer new possibilities for novel applications due to their amphiphilicity and stronger binding to fluid interfaces compared to homogeneous particles. Introducing surface anisotropy creates complexity as the orientation of interfacially bound particles affects interparticle interactions, a contributing factor to the microstructure formation. In this work, we have investigated the microstructure of JP monolayers formed at the air–water interface using particles with different degrees of amphiphilicity and examined the response of the networks to applied compressions. Our findings demonstrate that JPs amphiphilicity is a crucial factor governing their orientation at the interface, which in turn dictates the complexity of the capillary interactions present and the mechanical properties of the ensuing networks.
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Chemically active colloids self-propel by catalysing the decomposition of molecular ‘fuel’ available in the surrounding solution. If the various molecular species involved in the reaction have distinct interactions with the colloid surface, and if the colloid has some intrinsic asymmetry in its surface chemistry or geometry, there will be phoretic flows in an interfacial layer surrounding the particle, leading to directed motion. Most studies of chemically active colloids have focused on spherical, axisymmetric ‘Janus’ particles, which (in the bulk, and in absence of fluctuations) simply move in a straight line. For particles with a complex (non-spherical and non-axisymmetric) geometry, the dynamics can be much richer. Here, we consider chemically active helices. Via numerical calculations and slender body theory, we study how the translational and rotational velocities of the particle depend on geometry and the distribution of catalytic activity over the particle surface. We confirm the recent finding of Katsamba et al. ( J. Fluid Mech. , vol. 898, 2020, p. A24) that both tangential and circumferential concentration gradients contribute to the particle velocity. The relative importance of these contributions has a strong impact on the motion of the particle. We show that, by a judicious choice of the particle design parameters, one can suppress components of angular velocity that are perpendicular to the screw axis, or even select for purely ‘sideways’ translation of the helix.more » « less
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Abstract One of the major drawbacks for many biocatalysts is their poor stability under industrial process conditions. A particularly interesting example is the supply of oxygen to biooxidation reactions, catalyzed by oxidases, oxygenases or alcohol dehydrogenases coupled with NAD(P)H (reduced nicotinamide adenine dinucleotide phosphate) oxidases, which all require the continuous supply of molecular oxygen as an oxidant or electron acceptor. Commonly, oxygen is supplied to the bioreactor by air sparging. To ensure sufficient oxygen transfer from the gas to the liquid phase, stirring is essential to disperse the gas bubbles and create high gas‐liquid interfacial area. Studies indicate that the presence of gas‐liquid interface induces enzyme deactivation by protein unfolding which then readily aggregates and can subsequently precipitate. This contribution has examined the effects of stirring and the presence of gas‐liquid interface on the kinetic stability of water‐forming NAD(P)H oxidase (NOX) (EC1.6.3.2).
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