Hydrochromic materials have been actively investigated in the context of humidity sensing and measuring water contents in organic solvents. Here we report a sensor system that undergoes a brilliant blue-to-red colour transition as well as 'Turn-On' fluorescence upon exposure to water. Introduction of a hygroscopic element into a supramolecularly assembled polydiacetylene results in a hydrochromic conjugated polymer that is rapidly responsive (<20 μs), spin-coatable and inkjet-compatible. Importantly, the hydrochromic sensor is found to be suitable for mapping human sweat pores. The exceedingly small quantities (sub-nanolitre) of water secreted from sweat pores are sufficient to promote an instantaneous colorimetric transition of the polymer. As a result, the sensor can be used to construct a precise map of active sweat pores on fingertips. The sensor technology, developed in this study, has the potential of serving as new method for fingerprint analysis and for the clinical diagnosis of malfunctioning sweat pores.

Janus particles are colloids that have both hydrophilic and hydrophobic faces. Recent advances in particle synthesis enable the generation of geometrically and chemically anisotropic Janus particles with high uniformity and precision. These amphiphilic particles are similar to molecular surfactants in many aspects; they self-assemble in bulk media and also readily attach to fluid interfaces. These particles, just like molecular surfactants, could potentially function as effective stabilizers for various multiphasic systems such as emulsions and foams. In particular, just as the shape and chemical composition have a significant impact on the surfactancy of molecular amphiphiles, the ability to control the shape and wetting properties of Janus particles could provide a unique opportunity to control their surface activity. In this review, we first examine the recent developments in using amphiphilic Janus particles as colloid surfactants to stabilize multiphasic mixtures such as emulsions. These results have motivated a number of detailed investigations aimed at understanding the behaviour of Janus particles at fluid–fluid interfaces at the microscopic level, which we highlight. This review also discusses the importance of controlling the shape of Janus particles, which has a drastic impact on their behaviour at fluid interfaces. We conclude this review by presenting outlook on the future directions and outstanding problems that warrant further study to fully enable the utilization of Janus particles as colloid surfactants in practical applications.

We study the behaviour of Janus particles at an oil–water interface. Amphiphilic Janus particles exhibit attractive interactions at the fluid–fluid interface. The attractive interactions are likely due to the pinning of contact line around the diffuse boundary between the two hemispheres. The undulation of the three-phase contact line around Janus particles leads to quadrupolar capillary interactions, which we confirm by measuring the interparticle forces. We also show that Janus particles with two negatively charged-hydrophilic hemispheres show repulsive interactions.

The effects of salts and surfactants on the interaction force between colloidal polystyrene latex particles confined to a decane–water interface are measured directly using optical tweezers. After adding 0.25 M NaCl, 0.25 M NaCl and 0.1 mM sodium dodecyl sulfate (SDS) to the aqueous sub-phase, or 25 μM sorbitan monooleate (SPAN 80) to the decane super-phase, the strong repulsive force between particles is reduced and an attractive force becomes significant. The magnitude and dependence of the attraction on particle separation is consistent with a capillary quadrupole interaction. Similar interaction forces between polystyrene latex doublet particles at a pristine interface are measured, however, the anisotropic particles exhibit only a long-range attraction that is approximately two orders of magnitude stronger than spherical colloids. These results confirm the presence of long-range capillary attractions and provide a guide for manipulating colloidal interactions with additives or particle shape at fluid interfaces to control suspension structure and surface rheology.