Evolution of a colloidal critical state in an optical pinning potential landscape

Abstract

As a step toward isolating the influence of a modulated substrate potential on dynamics and phase transitions in two dimensions, we have studied the behavior of a monolayer of colloidal spheres driven by hydrodynamic forces into a large array of holographic optical tweezers. These optical traps constitute a substrate potential whose symmetry, separation, and depth of modulation can be varied independently. We describe a particular set of experiments, in which a colloidal monolayer invades the optical pinning potential much as magnetic flux lines invade type II superconductors, including cooperative avalanches, streaming motion, and a symmetryaltering depinning transition. The jammed intermediate state in this process resembles the critical state long associated with flux entering zero-field-cooled superconductors. By tracking the particles' motions, we are able to determine the microscopic processes responsible for the evolution of the colloidal critical state and compare these with recent simulations of flux-line dynamics.In the absence of an externally imposed potential energy landscape, the behavior of two-dimensional systems is wholly determined by interactions among the constituent particles. Examples of such systems include electrons on the surface of liquid helium, vortices in clean type II superconductors, and colloidal monolayers. Colloidal monolayers in particular have been studied as model systems whose phase behavior offers insight into the general mechanisms of structural phase transitions in reduced dimensionality. 1 In contrast, the majority of two-dimensional systems are strongly influenced by their constituent particles' interactions with substrate potentials. Adatoms adsorbed on crystal surfaces, 2 magnetic flux lines pinned in defected and patterned 3-8 type II superconductors, atoms imbibed into graphite intercalation compounds, 9 and charge density waves 10 all reflect such an influence with the appearance of different thermodynamic, dynamic, and kinetic phases and states.Understanding how substrates modify two-dimensional phase behavior is complicated in experimental studies by the comparative difficulty of quantifying and controlling substrates's properties. However, colloids' interaction with easily controlled external forces offers some possibilities. For example, a physically textured surface with micrometer-scale features can influence the free energy of a colloidal overlayer, either through electrostatic interactions or through entropic depletion attractions mediated by small dispersed particles. 11 Alternatively, a pattern of light applied to the sample can create a modulated potential energy landscape with which the colloid can interact. [12][13][14][15] We have utilized this approach by applying the holographic optical tweezer technique 16 -18 to create tailored optical potential landscapes for colloidal particles. This paper describes experimental observations of convection-driven colloidal transport into a large, initially empty array of discrete holographic optical traps. The...

Realizing Colloidal Artificial Ice on Arrays of Optical Traps

“…In addition, the colloidal interaction can be changed from nearest neighbor to longer range simply by adjusting the screening length. A variety of different static and dynamical trap geometries can be constructed with optical arrays [8], and colloidal crystallization and melting have already been demonstrated in square and triangular optical trap arrays [9,10]. It is also possible to make arrays with elongated traps that have a double well shape such that a single colloid can be located in either well [11,12].…”

Realization of Artificial Ice Systems for Magnetic Vortices in a Superconducting MoGe Thin Film with Patterned Nanostructures

“…Numerical simulations within the nonlinear Ginzburg-Landau theory reveal a square vortex ice configuration in the ground state at the half matching field and demonstrate similar characteristic features in the field dependence of the critical current, confirming the experimental realization of an artificial ice system for vortices for the first time. Artificial ice systems [1][2][3][4][5][6][7][8][9][10][11][12][13][14] that can have properties similar to atomic spin ices [15][16][17][18][19][20] have been gaining tremendous interest in recent years in areas ranging from solid state systems, magnetism, and soft matter. Among them the two-dimensional (2D) artificial spin ices created using e.g., nanomagnetic arrays [1][2][3][4][5][6][7][8] and charged colloidal particle assemblies [9][10][11][12][13][14] have opened a new avenue in the study of novel phenomena such as geometrical frustration [7,8,[15][16][17][18][19][20][21][...…”

“…Na, 74.40.Gh Artificial ice systems [1][2][3][4][5][6][7][8][9][10][11][12][13][14] that can have properties similar to atomic spin ices [15][16][17][18][19][20] have been gaining tremendous interest in recent years in areas ranging from solid state systems, magnetism, and soft matter. Among them the two-dimensional (2D) artificial spin ices created using e.g., nanomagnetic arrays [1-8] and charged colloidal particle assemblies [9][10][11][12][13][14] have opened a new avenue in the study of novel phenomena such as geometrical frustration [7,8,[15][16][17][18][19][20][21][22][23][24] which can elucidate, e.g., exotic spin states, [16] charge quantization in magnetic monopoles, [21,22] and mechanisms of high-T c superconductivity. [24] In artificial nanomagnetic square spin ices, [1,8] however, the ice rule states with spin arrangements following "two spins in, two spins out" orders …”

Nonlinear dynamics, rectification, and phase locking for particles on symmetrical two-dimensional periodic substrates with dc and circular ac drives

“…If the colloids are charged they can be driven with dc and ac electric fields. The most promising approach would be to use periodic arrays of optical traps [33][34][35][36] or dynamical optical trap arrays [11,13]. In this case colloids can be trapped at individual spots of laser light.…”