Chemokinesis-driven accumulation of active colloids in low-mobility regions of fuel gradients

Abstract

Many motile cells exhibit migratory behaviors, such as chemotaxis (motion up or down a chemical gradient) or chemokinesis (dependence of speed on chemical concentration), which enable them to carry out vital functions including immune response, egg fertilization, and predator evasion. These have inspired researchers to develop self-propelled colloidal analogues to biological microswimmers, known as active colloids, that perform similar feats. Here, we study the behavior of half-platinum half-gold (Pt/Au) self-propelled rods in antiparallel gradients of hydrogen peroxide fuel and salt, which tend to increase and decrease the rods’ speed, respectively. Brownian Dynamics simulations, a Fokker–Planck theoretical model, and experiments demonstrate that, at steady state, the rods accumulate in low-speed (salt-rich, peroxide-poor) regions not because of chemotaxis, but because of chemokinesis. Chemokinesis is distinct from chemotaxis in that no directional sensing or reorientation capabilities are required. The agreement between simulations, model, and experiments bolsters the role of chemokinesis in this system. This work suggests a novel strategy of exploiting chemokinesis to effect accumulation of motile colloids in desired areas.

AI-based inverse design for electrochemically controlled microscopic gradients of O2 and H2O2

“…In addition, the morphology of wire array electrodes impacts the generated H2O2 gradient. The characterized H2O2 gradients for k = (15, 4, 50), (30,3,50), and (15,4,20) (black, red, and blue in Fig. 3e, respectively) were noticeably different at the same Eappl = 0.5 V vs. RHE.…”

“…We also conducted a spot check for the predicted O2 and H2O2 microenvironments by experimentally validations. Pt-based wire array electrode with k = (46,6,20) was picked from Fig. 5c as a desirable geometry, experimentally prepared (Fig.…”

“…Despite the recent progresses [14][15][16][17][18] there remain major technical challenges, particularly in the achievable spatiotemporal resolution and high-throughput design of concentration profiles to suit a plethora of scenarios in microbiology. Approaches based on microfluidics and hydrogels have been able to achieve concentration gradients of O2 and H2O2 through the provision of either O2/H2O2 source 14,[19][20][21] , O2/H2O2 scavenging agents 15,22,23 , or a combination of both 24 across liquidimpermeable barriers such as agar layers or polymeric thin films. 25,26 Yet such approaches dependent on passive mass transport and diffusion across more than 10 2 μm are inherently incapable of achieving spatial features of less than 100 μm and temporal resolution smaller than ~10 1 seconds, the prerequisites to investigate microbiology at cluster or single-cell levels.…”

Excess Entropy Scaling in Active-Matter Systems

“…Active-matter systems feature discrete particles that can convert stored or ambient free energy into motion. − The transport properties of systems containing active particles can differ significantly compared to those of their inactive counterparts (see, e.g., the diffusion coefficient of a self-propelled microswimmer compared to that of a Brownian particle, − the onset of turbulence in active fluids at Reynolds numbers lower than those in classical turbulence, − or a variety of collective motion phenomena with no close inactive analogues , ). Advancements over the past decade in methods for synthesizing active particles − have enabled the realization of numerous active-matter systems, exhibiting a rich array of structural and transport phenomena. One particularly exciting engineering application of active particles is for modifying (and perhaps eventually controlling) the material and transport properties of the (inactive) fluid within which they are suspended, as has been explored in the case of diffusion, − viscosity, heat transfer properties, and phase behavior. − …”

Machine learning–based inverse design for electrochemically controlled microscopic gradients of O 2 and H 2 O 2

“…Despite recent progress ( 14 – 18 ), there remain major technical challenges, particularly in the achievable spatiotemporal resolution and high-throughput design of concentration profiles to suit a plethora of scenarios in microbiology. Approaches based on microfluidics and hydrogels have been able to achieve concentration gradients of O 2 and H 2 O 2 through the provision of either O 2 /H 2 O 2 source ( 14 , 19 – 21 ), O 2 /H 2 O 2 scavenging agents ( 15 , 22 , 23 ), or a combination of both ( 24 ) across liquid-impermeable barriers such as agar layers or polymeric thin films ( 25 , 26 ). Yet such approaches, dependent on passive mass transport and diffusion across more than 10 2 μm, are inherently incapable of achieving spatial features of less than 100 μm and temporal resolution smaller than ∼10 1 s, the prerequisites to investigate microbiology at cluster or single-cell levels ( 10 – 12 ).…”