Artificial micro heat engines are prototypical models to explore and elucidate the mechanisms of energy transduction in a regime that is dominated by fluctuations [1,2].Micro heat engines realized hitherto mimicked their macroscopic counterparts and operated between reservoirs that were effectively thermal [3][4][5][6][7]. For such reservoirs, temperature is a well-defined state variable and stochastic thermodynamics provides a precise framework for quantifying engine performance [8,9]. It remains unclear whether these concepts readily carry over to situations where the reservoirs are outof-equilibrium [10], a scenario of particular importance to the functioning of synthetic [11,12] and biological [13] micro engines and motors. Here we experimentally realized a micrometer-sized active Stirling engine by periodically cycling a colloidal particle in a time-varying harmonic optical potential across bacterial baths at different activities.Unlike in equilibrium thermal reservoirs, the displacement statistics of the trapped particle becomes increasingly non-Gaussian with activity. We show that as much as ≈ 85% of the total power output and ≈ 50% of the overall efficiency stems from large non-Gaussian particle displacements alone. Most remarkably, at the highest activities investigated, the efficiency of our quasi-static active heat engines surpasses the equilibrium saturation limit of Stirling efficiency -the maximum efficiency of a Stirling engine with the ratio of cold and hot reservoir temperatures T C T H → 0. Crucially, the failure of effective temperature descriptions [14-16] for active reservoirs highlights the dire need for theories that can better capture the physics of micro motors and heat engines that operate in strongly non-thermal environments.

The transformation of flowing liquids into rigid glasses is thought to involve increasingly cooperative relaxation dynamics as the temperature approaches that of the glass transition. However, the precise nature of this motion is unclear, and a complete understanding of vitrification thus remains elusive. Of the numerous theoretical perspectives 1-4 devised to explain the process, random first-order theory (RFOT; refs 2,5) is a well-developed thermodynamic approach, which predicts a change in the shape of relaxing regions as the temperature is lowered. However, the existence of an underlying 'ideal' glass transition predicted by RFOT remains debatable, largely because the key microscopic predictions concerning the growth of amorphous order and the nature of dynamic correlations lack experimental verification. Here, using holographic optical tweezers, we freeze a wall of particles in a two-dimensional colloidal glass-forming liquid and provide direct evidence for growing amorphous order in the form of a static point-to-set length. We uncover the non-monotonic dependence of dynamic correlations on area fraction and show that this non-monotonicity follows directly from the change in morphology and internal structure of cooperatively rearranging regions 6,7 . Our findings support RFOT and thereby constitute a crucial step in distinguishing between competing theories of glass formation.In a seminal paper dated nearly fifty years ago, Adam and Gibbs 8 associated the rapid growth of a supercooled liquid's relaxation time with a decrease in its configurational entropy S c . Within RFOT, S c is related to the number of metastable minima in the free energy landscape of the liquid that can be explored by the system at a given temperature. This theory further claims that the supercooled liquid freezes into a mosaic whose domains correspond to configurations in these metastable minima 1 . The typical domain size is expected to diverge at the 'ideal' glass transition temperature, where S c vanishes. The existence of a growing static 'mosaic' length scale that serves as a clear indicator of the glass transition is therefore intrinsic to RFOT (ref. 2), although a systematic procedure for measuring it from point-to-set correlations was established much later 9 . Since the findings of ref. 9, a variety of growing static length scales have been identified and computed in numerical simulations [10][11][12][13] . Of these, the point-to-set correlation length ξ PTS (ref. 10) is of central importance, as it follows directly from the mosaic picture. ξ PTS is measured by freezing a subset of particles in the liquid's equilibrium configuration, and examining their influence on the configuration of the remaining free particles. As such, when evaluated for appropriate pinning geometries, ξ PTS can provide an estimate of the typical domain size of the mosaic 10 . In addition, it has been shown analytically that a divergence in the relaxation time is indeed associated with a diverging ξ PTS (ref. 14). ξ PTS was first extracted in simulation...

Colloids as Models Colloids are often used as analogs for atoms in order to study crystallization kinetics or glassy dynamics using particles that are much easier to observe and that move on much slower time scales. Ganapathy et al. (p. 445 ; see the Perspective by Einstein and Stasevich ) consider whether the analogous behavior extends to the growth of epitaxial films, a technique that is used in manufacturing. Controlling the rate of addition of the colloidal particles allowed the mapping of diffusional pathways during film nucleation and growth on a patterned substrate. The same relationships used to describe atomistic growth could be applied to the colloidal systems, but certain growth barriers such as those found at step edges and corners were controlled by diffusion rather than energetics.

We study experimentally the glass transition dynamics in quasi-two-dimensional suspensions of colloidal ellipsoids, aspect ratio α=2.1, with repulsive as well as attractive interactions. For the purely repulsive case, we find that the orientational and translational glass transitions occur at the same area fraction. Strikingly, for intermediate depletion attraction strengths, we find that the orientational glass transition precedes the translational one. By quantifying structure and dynamics, we show that quasi-long-range ordering is promoted at these attraction strengths, which subsequently results in a two-step glass transition. Most interestingly, within experimental certainty, we observe reentrant glass dynamics only in the translational degrees of freedom.

We present direct experimental signatures of a nonequilibrium phase transition associated with the yield point of a prototypical soft solid-a binary colloidal glass. By simultaneously quantifying single-particle dynamics and bulk mechanical response, we identified the threshold for the onset of irreversibility with the yield strain. We extracted the relaxation time from the transient behavior of the loss modulus and found that it diverges in the vicinity of the yield strain. This critical slowing down is accompanied by a growing correlation length associated with the size of regions of high Debye-Waller factor, which are precursors to yield events in glasses. Our results affirm that the paradigm of nonequilibrium critical phenomena is instrumental in achieving a holistic understanding of yielding in soft solids.