NOVELTY - Making hierarchical structures comprises: preparing a mixture of relatively smaller metal particles having a size below a threshold for nanowire formation and of relatively larger metal particles having a size above the threshold for nanowire formation; and oxidizing the mixture at a selected temperature and for a selected time, by which the relatively smaller particles sinter and nanowires grow on the relatively larger particles, thus creating tunable hierarchical structures.    USE - The method is useful for making hierarchical structures, which are useful: in thermal capacitor for thermal storage/regulation; in ultracapacitor; in thermal battery; in thermal interface material for spray impingement of droplets in a heat pipe; for spray cooling; in high-porosity hydrocarbon catalyst; in gas sensor; in contrast agent for MRI; heat pipe; as porous wicking structure in geothermal heat pipe; for carbon sequestration; and as battery anode material (all claimed).    ADVANTAGE - The method provides the hierarchical structures, which: provide more efficient boiling surface by tuning the size of the surface structures to optimize for high capillarity forces, bubble nucleation, superhydrophilicity, and escape of vapor; exhibit high surface area (thus suitable for catalysis) and desired thermoelectric properties; and prevent particle agglomeration with secondary nanowire structure.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) controlling extent of nanowire growth on a metal particle, comprising: selecting a starting particle size with respect to a size threshold for nanowire growth; and oxidizing the particle at a selected temperature and for a selected time by which a desired extent of nanowire growth is achieved;    (2) making a hollow structure, comprising thermally oxidizing a metal particle having a selected diameter;    (3) an article comprising nanowires extending from the surface of a particle, where the particle includes elements having multiple oxidation states;    (4) hollow metal particle made by oxidation, where the metal particle has a diffusivity of the metal through its oxide greater than the diffusivity of oxygen through the oxide;    (5) making a material or coating, comprising simultaneously sintering/oxidizing an unoxidized particle in ambient air;    (6) forming an oxidized network of partially sintered metal particles, comprising: sintering particles in vacuum, in an inert atmosphere or in a relatively non-reactive gas to initiate necking; and sintering the particles in air to form nanowires extending from the surface of the particle while retaining high thermal conductivity;    (7) ultracapacitor comprising material made by the method, where the ultracapacitor is filled with a dielectric material;    (8) thermal battery comprising structures made by the method, where the thermal battery is filled with a selected material to create a tunable thermal battery;    (9) thermal interface material comprising the structure made by method, where the thermal interface material is filled with a high thermal conductivity material;    (10) boiling surface comprising structures made by the method, where the size of the surface structure is optimized for high capillarity forces, bubble nucleation, superhydrophilicity and escape of vapor;    (11) the structure made by the method;    (12) particle comprising a metal oxide outer shell with metal oxide wires extending from the outer shell; and    (13) multiscale structure comprising particles above and below a critical size, where the particles above the critical size have wires extending from the particle surface.