How does a woodpecker work? An impact dynamics approach

To understand how a woodpecker is able accelerate its head to such a high velocity in a short amount of time,a multi-rigid-segment model of a woodpecker＇s body is established in this study.Based on the skeletal specimen of the woodpecker and several videos of woodpeckers pecking,the parameters of a three-degree-of-freedom system are determined.The high velocity of the head is found to be the result of a whipping effect,which could be affected by muscle torque and tendon stiffness.The mechanism of whipping is analyzed by comparing the response of a hinged rod to that of a rigid rod.Depending on the parameters,the dynamic behavior of a hinged rod is classified into three response modes.Of these,a high free-end velocity could be achieved in mode II.The model is then generalized to a multihinge condition,and the free-end velocity is found to increase with hinge number,which explains the high free-end velocity resulting from whipping.Furthermore,the effects of some other factors,such as damping and mass distribution,on the velocity are also discussed.

Cellulose Nanocrystals-based Chiroptical Materials

Chiroptical materials are widely used in photonic devices,enantioselective catalysis and bio-sensors.Cellulose-base chiroptical materials with multilength scale structural hierarchy and unique light manipulation ability found in nature provide inspiration for materials design.Cellulose nanocrystals(CNC)display twisted rod morphology and hierarchical chirality.Leveraging the evaporation-induced self-assembly of negatively charged CNC,a broad realm of CNC-based chiroptical materials featuring one-dimensional photonic bandgap and novel chiroptical properties have been developed,which are of scientific and technological significance.Here we presented a brief overview on CNC-based chiroptical materials by evaporation-induced self-assembly,showed energy and chirality transfer in a host-guest environment leading to photonic bandgap modulation of optoelectronic properties,outlined novel chiroptical phenomena and their underlying principles,and demonstrated the application potentials of the CNC-based chiroptical materials.

Adaptive Prescribed Performance Control for Flexible Spacecraft with Input Saturation and Actuator Misalignment

In this paper, a flexible spacecraft attitude control scheme that guarantees vibration suppression and prescribed performance on transient-state behavior is proposed. Here, parametric uncertainty, external disturbance, unmeasured elastic vibration, actuator saturation, and even configuration misalignment are considered. To guarantee prescribed performance bounds on the transient-and steady-state control errors, a performance constrained control law is formulated with an error transformed function. An elastic modal observer is employed to estimate the unmeasured flexible modal variables, and a command filter is adopted to avoid the tedious analytical computations of time derivatives of virtual control inherent in the control design. Subsequently, a novel auxiliary system is developed to compensate the adverse effects of the actuator saturation constraints, and a compensation term is integrated into the control law to tackle the configuration misalignment. A comparative simulation study is carried out to illustrate the effectiveness and advantages of the proposed approach.

Optically Ambidextrous Reflection and Luminescence in Self-Organized Left-Handed Chiral Nematic Cellulose Nanocrystal Films

Defect-embedded chiral nematic organizations are attractive due to their light manipulation ability that is crucial for photonic applications.Self-assembly of cellulose nanocrystals(CNC)forms a left-handed chiral nematic structure,which makes simultaneous reflection of left-handed circularly polarized(LCP)and right-handed circularly polarized(RCP)light a challenging task.Herein,we present unprecedented evidence that self-organized CNC films with left-handed helical sense and photonic bandgaps(PBG)display ambidextrous CP light reflection with peak reflectivity up to 68%and transform spontaneous photoluminescence to RCP and LCP luminescence with respective|glum|values up to 0.796 and 0.120.These ambidextrous chiroptical properties originate from a left-handed chiral nematic structure consisting of a nematic-like phase,acting as a half-wave retarder,sandwiched between two PBG layers.The latter are spontaneously intercalated into the left-handed helicoids by rapid gelation of CNC colloidal suspensions.We demonstrate that the ambidextrous chiroptical properties can be manipulated by changes in evaporation temperatures,concentrations,and characteristics of CNC suspensions.We showcase the potential of defect-embedded chiral nematic CNC films for the sustainable and scalable development of anti-counterfeiting optical labels.