Concentration gradients play a critical role in embryogenesis, bacterial locomotion, as well as the motility of active particles. Particles develop concentration profiles around them by dissolution, adsorption, or the reactivity of surface species. These gradients change the surface energy of the particles, driving both their self-propulsion and governing their interactions. Here, we uncover a regime in which solute gradients mediate interactions between slowly dissolving droplets without causing autophoresis. This decoupling allows us to directly measure the steady-state, repulsive force, which scales with interparticle distance as F ∼ 1/r 2 . Our results show that the dissolution process is diffusion rather than reaction rate limited, and the theoretical model captures the dependence of the interactions on droplet size and solute concentration, using a single fit parameter, l = 16 ± 3 nm, which corresponds to the length scale of a swollen micelle. Our results shed light on the out-of-equilibrium behavior of particles with surface reactivity.

We describe colloidal Janus particles with metallic and dielectric faces that swim vigorously when illuminated by defocused optical tweezers without consuming any chemical fuel. Rather than wandering randomly, these optically-activated colloidal swimmers circulate back and forth through the beam of light, tracing out sinuous rosette patterns. We propose a model for this mode of light-activated transport that accounts for the observed behavior through a combination of self-thermophoresis and optically-induced torque. In the deterministic limit, this model yields trajectories that resemble rosette curves known as hypotrochoids.

Brownian vortexes are stochastic machines that use static non-conservative force fields to bias random thermal fluctuations into steadily circulating currents [1,2]. The archetype for this class of systems is a colloidal sphere in an optical tweezer [1,3,4]. Trapped near the focus of a strongly converging beam of light, the particle is displaced by random thermal kicks into the nonconservative part of the optical force field arising from radiation pressure [5], which then biases its diffusion [1,3]. Assuming the particle remains localized within the trap, its time-averaged trajectory traces out a toroidal vortex. Unlike trivial Brownian vortexes, such as the biased Brownian pendulum, which circulate preferentially in the direction of the bias, the general Brownian vortex can change direction and even topology in response to temperature changes. Here we introduce a theory based on a perturbative expansion of the Fokker-Planck equation for weak non-conservative driving. The first-order solution takes the form of a modified Boltzmann relation and accounts for the rich phenomenology observed in experiments on micrometer-scale colloidal spheres in optical tweezers.

We show that a bright-field defocused microscope is effectively a phase-contrast microscope, but with advantages over the conventional one and maintaining the same optical resolution. In a multilayered transparent object, the height amplitude (static and dynamic) of each interface can be measured separately with nanometer sensitivity. By scanning the position of the objective focal plane in relation to the surfaces of a red blood cell, we obtain quantitative information on height fluctuations from each surface individually, which can be analyzed with our model of a defocused microscope and compared with theoretical models.

Colloidal spheres synthesized from polymer gels swell by absorbing molecules from solution. The resulting change in size can be monitored with nanometer precision using holographic video microscopy. When the absorbate is chemically similar to the polymer matrix, swelling is driven primarily by the entropy of mixing, and is limited by the surface tension of the swelling sphere and by the elastic energy of the polymer matrix. We demonstrate though a combination of optical micromanipulation and holographic particle characterization that the degree of swelling of a single polymer bead can be used to measure the monomer concentration in situ with spatial resolution comparable to the size of the sphere.Stimulus-responsive colloidal particles 1 respond to physical or chemical changes in their environment through measurable changes in their own physical properties. Such particles have proved useful in a wide range of applications, ranging from drug-delivery systems to probe particles for sensors. Monitoring probe particles' responses can be challenging, particularly for local probes involving changes in isolated particles. Here, we demonstrate that in-line holographic microscopy can be used to gauge the swelling of individual micrometer-scale polymer-gel spheres in situ and thus to measure the local concentration of selected chemical species, with excellent spatial and temporal resolution. The key to this technique is the ability of quantitative holographic video microscopy 2 to report the probe sphere's radius with nanometer precision while simultaneously monitoring its refractive index with part-per-thousand resolution.To demonstrate the holographic concentration probe, we combine holographic micromanipulation 3 with holographic video microscopy 2,4 to measure concentration profiles of solubilized silicone oil in water. Our probe particles consist of polydimethylsiloxane (PDMS) synthesized by base-catalyzed hydrolysis and copolymerization of difunctional dimethyldiethoxysilane (DMDES) and trifunctional methyltriethoxysilane (MTES) 5 . Synthesis and characterization of these particles is described elsewhere 6 . When dispersed in pure water, these spheres have a nominal radius of a 0 = 1 µm, as measured by scanning electron microscopy and in situ holographic characterization 2,6 . Trifunctional groups act as crosslinkers for the PDMS gel, and the particles used in this study have crosslinker fractions of ξ = 0, 0.4, and 0.8.PDMS gels absorb silicone oil and thus swell in the presence of monomeric DMDES to a degree that depends on the monomers' concentration in solution. Measuring the sphere's radius through holographic microscopy then provides a means to monitor the concentration of dissolved DMDES in real time.Our system, depicted schematically in Fig. 1, consists of a mixture of PDMS spheres and silica spheres (Bangs Laboratories, Catalog number SS04N) dispersed in a 0.1 M solution of aqueous ammonium hydroxide (Fisher Scientific). This solution fills half the length of a 2 cmlong rectangular capillary tube with 50...